It's funny, I was also trying to find out which line causes the issue, and I managed to call up to
const HttpResponse = container.resolve("HttpResponseUtil");
anything below this will break the flea.
As I removing mods to find out which mod breaks the flea, I found out that another mod is also breaking it. And it is also using PostDBLoad, so high chances that PostDBLoad is bugged.
Thanks mate! I will stay with the broken flea for while then