How do you mean FlatAssembler is enrolled in a class? He is an amateur who's not willing to learn why his theories may be wrong. When he faces some opposition, he runs away from the forum and makes some ugly website about his ideas.

And why do you keep insisting that the Proto-Indo-European hypothesis has the same scientific value as the theory of evolution? Social sciences don't appear to be real sciences. Trusting the mainstream social sciences has brought us things such as socialism and communism. Seems to me that I am better off thinking with my own head than trusting you guys.

FlatAssembler linked me to his webpage about Croatian toponyms (which links to this thread) on another forum when I told him he knows too little about Croatian history.
I can say that the Croatian nationalists are crazier than most of the conspiracy theorists. First they deny the obvious fact that Croatian and Serbian are the same language. They say they are "closely related Slavic languages". Then FlatAssembler goes as far as saying that the Croatian toponyms aren't Slavic, but come from some unattested language. The amount of mental gymnastics it takes to invent a whole language, completely different from Croatian, from which the Croatian toponyms supposedly come, is impressive. I don't know what will be the next level.

BTW, why the hell did it take this forum more than a day to send me an activation e-mail so that I can post here?

OK, now, have you even skimmed through the FlatAssembler's webpage about Croatian toponyms?
http://flatassembler.000webhostapp.com/toponyms.html
His proposal is obviously motivated by the Croatian right-wing politics. Or are you really that unaware of the Balkan politics?
You think that just because you know some linguistics (whatever that actually meant), you can evaluate theories about Croatian history? Do you even know some (Serbo-)Croatian? Because many of the toponyms he cites are perfectly explicable using it. The name of the river Vuka, for instance, obviously comes from the common Serbian personal name Vuk. He just doesn't want to admit that it's Serbian and therefore that it should belong to Serbia. This has nothing to do with linguistics, this has to do with politics.

Oh, OK, I see my posts get deleted here if I say what I think about the important stuff. Let's try, like FlatAssembler, you know, discuss politics without directly mentioning it.

So, FlatAssembler suggests that the name of the city of Zagreb, the capital of Croatia, comes from the Illyrian word he reconstructs as *Dzigurevos, supposedly meaning "God's hill". Why is that more credible than saying that the name "Zagreb" comes from the earlier name "Serbinovo"? Because any competent historian will affirm you Zagreb was previously known as "Serbinovo".

Well, yes, FlatAssembler did include tables on his web-page explaining what he supposes to be the sound changes from Indo-European to Illyrian (I believe he also posted that one here), from Classical Latin to the Late Latin dialect he supposes to have been spoken in Croatia and from Old Croatian into Modern Croatian. But why should we trust him? After all, he had quite a lot of time to make those stuff up in order to make his theory apparently coherent, right?
And I suppose it seems apparently coherent... until you know a bit more. For instance, that there was a city in ancient times known as Serbinon (and later as Sorba, Soroba, Sogora, Zbreg and finally as Zagreb), which all the historians I've asked agree was modern-day Zagreb. Doesn't that strongly suggest that Zagreb is a Serbian city that was stolen from Serbia by the Croats? And, as FlatAssembler also notes, there was also a river named Serapia, probably the river now known as Bednja. Yes, you can search for alternative etymologies (you can claim whatever you want about a supposed ancient unattested language), but come on now! Serbia, Serbinon, Serapia... Don't you see the connection here?
But, yeah, you are not from Balkan, so it's not important to you.

So, what is that approach you suggest? I've included a list of sound changes I propose in my last post.
And I think my approach is even more scientific than the FlatAssembler's one because that names which I take to be previous names for Zagreb are actually attested, the name "Serbinon" is attested on the Ptolemy's map, the name "Soroga" is mentioned in "Rerum Hungaricarum", and the other names similarly so.
FlatAssembler's proposed ancient name for Zagreb, "Dzigurevos" (supposedly meaning "God's hill"), as far as I know, hasn't been attested anywhere. Here is what he wrote exactly:

The name Zagreb itself probably means "the God's hill", from the Illyrian word Dzis (God), mentioned on the Messapian inscriptions, and a word related to Proto-Slavic *gora and Sanskrit giri. So, the Illyrian name might have been *Dzigurevos, borrowed into Old Croatian via Vulgar Latin.

He also appears to imply the way the name changed can be explained using the table he made (I linked to it in my last post), even though I find it very hard to believe languages actually behave that way. He derives the rules in that table from some words and place names he claims to have gone from Latin to Croatian early, but I am pretty sure that if you tested those rules with more such words, they will fail to correctly predict how the shapes of the words changed more often than not. And I am also pretty sure no linguist specializing in Serbo-Croatian would affirm you the rules in the table are correct.
So, which approach do you think is more scientific?

His proposal is obviously motivated by the Croatian right-wing politics.

I'm not suggesting anyone's politically-motivated arguments are better than anyone else's.

Why would my hypotheses be politically motivated? I just don't get what you are trying to say. I am not a fan of the Croatian right-wing politics, I am a libertarian. And, even so, somebody motivated by the Croatian right-wing politics would probably claim (without evidence) that the toponyms come from Croatian, not that they don't come from Croatian. Daniel, do you get what that guy is talking about?

The name of the river Vuka, for instance, obviously comes from the common Serbian personal name Vuk.

Then, how it is that the name Vuka is attested back in antiquity, way before there were Serbs (or even Slavs) there, as Ulca? Also, how is "Vuk" an exclusively Serbian name? It's a word for "wolf" in both Croatian and Serbian.
Also, a bit of an off-topic remark, but if the Serbs there would be so much better off had Serbia conquered Vuka, why don't they move to Serbia then? You know, to a place where everyone shares their language and presumably also their religion? Could it be that they feel they are less oppressed by the Croatian government (which, I agree, doesn't treat people of different nationalities fairly) than they would be by the government of the Serbian president Aleksandar Vucic?

the name "Serbinon" is attested on the Ptolemy's map,

Historians don't quite agree where Serbinon was, but the map you've linked to doesn't quite suggest it was near modern-day Zagreb. Nor does, as far as I am aware of, any Croatian historian agree that Serbinon was there.
Zagreb is on the Sava river, Serbinon (at least on the map you've shown here) is on the northern bank of the Drava river (that is, somewhere in south-west Hungary). Zagreb would be somewhere near Siscia and Segestica and north-west of them, not north-east (unless you assume all the traditional ubications are somehow wrong).
Besides, the right inference from the data you've shown is that Serbinon was named after the Serapia river (for it was near its confluence, at least on the map you've linked to), not after the Serbs (who, according to the mainstream history, never were there to begin with, and were not even close to there until at least the 5th century CE).

I find it very hard to believe languages actually behave that way.

How is anything else even conceivable? Sound changes that would, for example, randomly (without a phonological rule) change some 'd'-es to 't'-es, some 'd'-es to 'z'-es and left some 'd'-es unchanged would make a language incomprehensible rather quickly. Or do you think that many words in languages come into being without etymologies, that many words are formed just by picking random sounds? That would also make a language incomprehensible.

I am pretty sure that if you tested those rules with more such words, they will fail to correctly predict how the shapes of the words changed more often than not.

So, you are not comfortable with hypotheses that can be hypothetically falsified? Sorry, that's what science is all about.

And I am also pretty sure no linguist specializing in Serbo-Croatian would affirm you the rules in the table are correct.

Have you tried asking some? Chances are, he or she knows about it less than I do and will be impressed by my work.

So, which approach do you think is more scientific?

Isn't it obvious by now? I'm basing my explanation that "Zagreb" comes from Illyrian *Dzigurevos on a set of potentially falsifiable hypotheses, presented in that table on my web-page. You are running away from making falsifiable hypotheses about sound changes because "I don't think that's how languages work.".