Calendar entries disappear after loading on Thunderbird start

Using Tunderbird 128.8.1esr (64-Bit) on Windows 11 Since today, my externally bound-in calendars (not google) stopped working. When i start Thunderbird, the calendars load as usual and all appointments show up, but after a few seconds, all calendars disappear. I can neither add new appointments nor see the current ones. I am using no addons, so I guess the calendar app is lightning Additionally, this seems to be not the first time, that a thunderbird update causes problems with the calendar (that is delivered within the same software-suite!) as i have seen multiple versions of the same post with different build numbers, all with the "solution" to reinstall. Thit can't be the goal of QC.