Issue with user's mailbox

[Zolla715]

Hi there

A user has had the same mailbox for a few years now. Standard M365 tenant with nothing fancy within Exchange. Over the last two weeks the user's mailbox is taking 3-5 minutes to connect. 

Trouble shooting done:
Cache within Outlook turned off.
New mail profile created within Windows 10.
Credential manager cleared.
Tried on new device, same issue
Removed addins
Started in safe mode
When connecting the user's mailbox to my Outlook desktop app, the app freezes and crashes.

Anything I could be trying?

Thanks

[#2]

I would check if the users has access to additional mailboxes like shared or resource, remove all access and try again. This may be caused by a corrupt shared mailbox. I have seen the behavior.

[#3]

Good idea, I'll check and reverty findings