You should be able to enable detailed logging in the EWS Retrieval (or whichever protocol you're using in this case) and find more detail about what was being imported when it failed. It may not give you the specific message necessarily but should get you in the ballpark. I have escalated a feature request to add a debug option of sorts that can be used for cases like this where initial import failed for any reason though!

Hello Kyle

Thanks for your feedback. Yes now in retrospect the detailed logging from EWS unfortunately doesn't help me anymore. I think it would be great if you could also activate that the import mail that is triggered by the system also has a file attachment with all failed mails, calendars, contacts, etc., preferably in the form of a CSV.

That way, every user can get to the bottom of it themselves without having to bother us in support.

Thanks and regards

Roger