Ok, so having the quickest look at the code of how a module is created I made a quick hack to "module.json", so that it at least is possible to enable it in my ongoing campaign. It probably will create a bunch of other issues down the line, this isn't fixing the actual problem, only the symptom, but if anyone else is desperate it might be worth it whilst knowing those trade-offs.
Open the module.json (in the root folder of the module, wherever you have your modules, mine was in "C:\Users\mathi\AppData\Local\FoundryVTT\Data\modules\symbaroum5ecore" on a Win machine) and change the value of "compatibility" (far down in the file) "maxiumum", which has a value of "2.9.9". Change the "2" to a "3", so it says "3.9.9".
It pretty much says that the module is compatible up to that version. Basically lying. But hey, now I can open my world and at least see the content. We will be forced to do todays gaming session completely analogue though as I don't trust all calculations and other tweaks are correct, but now I can at least show the maps I have drawn in Dungeondraft and imported in to FoundryVTT, on the TV we have on our gaming table.
Also, as long as the core module is enabled you can now again go in on every token and pick the correct character sheet, i.e. the old one, which has corruption etc on it.
The downside of doing it like this is that you will not get a notification for when the core module ACTUALLY is updated, as in properly updated, but as that seems highly uncertain right now it might be worth setting a reminder for checking that manually until then, and just keep gaming in the meantime.
