Stopped syncing with Microsoft account.

I only have one authorised Microsoft account that used - for years - to sync without any problems. Since about one months, the sync constantly fails. Both, on the Windows version of OneCal and the Android version. It correctly syncs the first time after reconnection, but then fails to sync updates of the calendar afterwards. Uninstalled, reinstalled; disconnected, reconnected accounts - no improvement. Only syncs once when newly reconnected. That's it. Not impressed.