Can't set Outlook Delegation

First of all I had a user that couldn't set the delegation on his Mail Account.
Error we were facing was that the delegates were not correct and couldn't be saved

There was no option to delete or to add users to the delegation, so I had a look in the OWA and found some users that weren't visible in de Outlook Client, but still couldn't add new users or edit existing users.

So I had a look with PowerShell on the Exchange Online Server and found out that there were some users still having rights on the Mail Account but didn't exists anymore on the network.

Get-MailboxFolderPermission -Identity <MailAccount>:\calendar

So after deleting those users from the Mail Account he could add and edit his delegations again.

Remove examples

Remove-MailboxFolderPermission -Identity [email protected]:\"Postvak In" -User [email protected]

Remove-MailboxFolderPermission -Identity [email protected]:\"calendar" -User [email protected]