Hi Vero,
as stated above - reindexing and widening of search scope did not help at first (nor did adding a second user and then deleting it again which another thread suggested).
It is crucial, though, to actually restart your mac after rebuilding the index was finished. Once I rebooted, everything turned back to normal, even my file search workflow started working again. Thanks for the help, sorry for the fuzz.
Not 100% sure, what steps where actually necessary to fix the issue, but don't forget a final reboot 🙂