I have to start Nextcloud several times manually each time after booting Windows. When it has started it works OK.
There is an error in the Application logfile: Event 1000 with exception code 5. Module ig4icd64.dll.
I disabled the autostart of Intel’s peristence module. So far, Nextcloud again starts automatically.
It unfortunately was a onetime joy. Does no more autostart, even multiple manual starts fail. After 5 or 6 tries, the software starts.
It seems that clearing the cache and reconnecting the Nextcloud desktop app to the server was the solution.