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.
Solution was only temporary. Problem persists with the same Event as in the OP.