Synced Exchange calendars (and events) disappear

[jfabini]

My TB client synchronizes Calendars with two Exchange instances (one on-prem, one O365). Happened now with TB 140.1.1 esr, applies to earlier releases, too - happened to me on Mac and on Linux.

For both instances I noticed the same issue: all calendar entries stored in an Exchange calendar disappear. Moreover, the entire Calendar entry and checkbox in the TBSync dialog (normally shown as "available resources") disappears - even though contact, task and other resource entries are visible (unchecked, I do not sync them - see Figure 1 below)! If I disable the account by unchecking the "enable and synchronize this account" box and enable it again, the Calendar checkbox entry is shown as resource as it should (Figure 2). After checking the Calendar checkbox (Figure 3) and force-synchronizing the calendar (Figure 4), everything works as expected - until the next disappearance. The image sequence below shows the strange behaviour. I've no clue at all why this happens, no deterministic patterns or actions recognizable (irregular intervals).

Figure 1: Resources: Calendar line disappeared. Annoying is that there is no error message or notification to warn the user about the disappeared calendar and potentially missing calendar entries on the daily agenda. Figure 2: After disabling and re-enabling the exchange account, the calendar resource is shown again, as it should. Figure 3: Activate the calendar Figure 4: Sync the calendar - now all entries are shown, again.

Any clue what could be the reason and in particular how to prevent these issues?