You are absolutely right....I'm feeling like an idiot, really! For weeks I've tried to change every single character in nginx configuration and your first tip to look at the request header was worth the number of characters in gold ;) wsc is indeed the default prefix for Woltlab Suite and indeed, the first configuration was with the sub domain and not the standalone domain.

I am sorry to wrongly suspected Nginx to be the source of this problem and appreciate your help very much, it really killed a lot of sleep time.