IMAP client errors retrieving mailbox content

Guys, I'm trying to access GroupWise accounts via an Outlook 2016 client, using IMAP as the server connector.

It works well, I am only trying to read data, not syncing data back into GroupWise or anything, but I consistently get sync errors and the item count in GroupWise does not match that in Outlook.

Sync Errors are shown in Outlook - Error Synchronizing folder. [800CCCD2-0-0-560]

So for example, GroupWise may show 7000 items in a folder, in Outlook you show 6943. And you see sync errors. It's the same error number each time, and it's happening on many mailboxes.

Any tips or ideas at all? Any likely candidates for items that Outlook won't like? Something IMAP has issues processing perhaps?

Thanks in advance. Alan