[@leanderbraunschweig Participant 13 years, 1 month ago]

Already did (re-)save the permalinks settings (see original post). Did it again right now just to make sure & still the same problem…

What to do? Which file / script / logic could be responsible?

When issuing a search-query, this is the “real” URL, which gets called first (before rewriting):
/search?action=bbp-search-request&bbp_search=my-query << No problems here, right?

FYI: I deactivated all the other plugins – no effect, the error persists.