Outlook Keeps popping up with enter username Password

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Hi

We have a staff member here who have their own mailbox along with an additional shared mailbox which they use

 

Since a few days ago each time the user access their Outlook a pop up appears asking user to enter their username and password which is linked to the name of their additional mailbox

 

I can't figure out why this is happening

 

I've tried the following

 

- On Exchange checked the user has permissions to that mailbox which they have

- Tried deleting their Outlook Profile and create a new one even at the creation of deleting their profile the pop up appears then, i can just by pass it by pressing escape and their main mailbox loads fine

 

 

but every so often it pops up asking to enter username and password for the additional mailbox

 

 

Anyone seen this before or any suggestion?