Cannot edit calendar items after a few hours

Hi, I have a strange experience with thunderbird's calendar: an hour or two after i created a new appointment in my calendar, i cannot change this appointment anymore. When i do so, this appointment is not saved anymore when i click on "save & close". When i alter and save this appointment within the hour it works normally and the item is saved as expected. I searched the settings, but nothing could be found concerning this issue. My OS: Ubuntu 24.04.2 LTS. Thank you very much for your efforts. Wyl. Holland.