Incorrect sync for calendar & tasks

I have the current version of Nextcloud running on a share hosting from Uberspace.
I use the calendar including tasks there with 3 people and various devices (iPhone, iPad, Mac). We use the standard calendar and reminder apps on the devices. Thunderbird is also installed on the Mac.
Unfortunately, the sync has not been working properly for about 1 year. Calendar entries are not displayed on some devices. Just like tasks. Checking off tasks is also sometimes not synced correctly. Everything is correct via the web interface, so I assume that the devices are not aware of the changes.
If I change an unsynced calendar entry on another device, it then also appears on the device where it was not initially displayed.
What could be the reason for this? Can I log the syncing of the devices?
Thanks for the help :slight_smile:

No one has an idea?

All I can add:
Same with me!

It’s driving us crazy…