Microsoft Outlook 365 sync issues on a Mac

I am using Microsoft Outlook 365 on a mid-2011 Mac running High Sierra.

Everything is working fine except I get sync issues on two specific email addresses every time Outlook does a mail fetch. I simply cannot trace where this is coming from. I have attach screenshots of the two issues. Two diffierent email addresses - two different error codes. The specific codes do not show any results on a google search.

The issue does not appear on the Windows version of Outlook 365 on my PC.

Can anyone throw any light on the solution to this?

Thanks,

Geoff