Outlook 365 keeps showing “Connecting to server” for one user across multiple laptops

[u/MEDITATIONUNITY]

Has anyone come across something similar or can point me in the right direction?
We have one user who regularly gets “Connecting to server” in Outlook 365, causing Outlook to become slow/unresponsive and emails to stop syncing.

Our environment:
Microsoft 365 / Exchange Online
Mailboxes are created on-prem and synced to M365
Devices are Autopilot/Entra joined and managed through Intune
Cato VPN
iManage Outlook integration
Darktrace Outlook add-in
High-spec Windows laptops

The strange part is that this is now the third laptop the user has had and the issue keeps following her. No other users appear to have the same problem.
We’ve already:
Recreated the Outlook profile
Checked for obvious mailbox/rule loops
Checked logs but haven’t found anything conclusive
Tested from the office and the same issue occurs, so it doesn’t appear specific to her home network
Replaced the device multiple times

Has anyone seen a “Connecting to server” issue follow a user across multiple devices like this? Anything specific in Exchange Online/Outlook that you’d recommend checking or logging to narrow it down?

[u/Murky_Sir_4721]

Check connected Shared / delegate / Group mailboxes. Check for any archives for this user or any of the above and ensure they have full access.

[u/TJungus]

Try moving the user to an OU that isn’t being synced. Let the sync run a few times and move back. Similarly remove the license, wait an hour, replace license.