If you subscribed to the Timebutler calendar in a newer Outlook calendar, automatic calendar 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 process it. Unfortunately, other calendars do not always adhere to the synchronization standard and expect a format that is actually invalid. Timebutler therefore provides the synchronization in a format that deviates slightly from the specification so that other calendars can perform the synchronization. This format triggers warnings in Outlook. However, these are only warnings, and Outlook still processes the synchronization and imports the absence entries correctly.
So synchronization in Outlook works without any issues, and you can simply ignore these messages.
If you still experience synchronization issues contrary to expectations, read more here about [Causes of errors in calendar synchronization(/frequently-asked-questions/calendar/sync-with-calendar-causes-of-errors/).