Hi Fred,
I read the post you provided and found that the customer opened a service request and finally he/she didn’t keep replying to the SR engineer.
For the issue you encountered, I tried to reproduce on my side and to no avail.
Currently, since the post is kind of relatively "long", to understand your exact scenario better, could you help confirm the following points?
1. Do the imported calendars show properly in OWA (Outlook Web App)?
2. You mentioned “his two Mac computers”, is his Office 365 account connected to Outlook on both the Mac?
3. If so, does the error exist in both Outlook? Are Outlook the same version on the two Mac?
4. Do other internal users encounter the same error?
5. To determine whether it's related to the PST file, could you provide the PST file by sending an email with the PST file as the attachment (I've sent you a private message (your details > Private messages) including my private email address)?
In addition, as you mentioned, the sync issues between the calendars on his two Mac computers may be related to the error. Let’s temporarily bypass it.
Thanks,
David