If you have subscribed to the Timebutler calendar in a newer Outlook calendar, the automatic synchronization in Outlook can trigger warning messages.
We are aware of these warnings, and they are not a problem.
For technical reasons, we designed the calendar sync transfer so that calendars other than Outlook can also process it. Unfortunately, not all calendars adhere to the synchronization standard, and some expect a format that is actually invalid. Timebutler therefore provides the synchronization in a format that deviates slightly from the standard so that these other calendars can synchronize as well. This format triggers warning messages in Outlook. However, these are only warnings — Outlook still synchronizes and imports the absence entries correctly.
Synchronization in Outlook works without issues, and you can simply ignore these messages.
If you still experience synchronization problems contrary to expectations, read more here about causes of errors in calendar synchronization.