I feel guilty for solving my own questions after 1-2 days, after some better search, but who knows, maybe it helps someone else, too…
So, the for me relevant logseq metadata was saved in ~/.logseq ( C:\Users\<YOUR_USERNAME>\.logseq in Windows), as answered here: How do I fully delete Logseq + all application metadata/app data? - #2 by Bad3r and here: Empty loading screen on app launch · Issue #3749 · logseq/logseq · GitHub
After deleting that folder and starting logseq, I got a beautiful fresh logseq.