We had a user who had a corrupt folder
AppData\Roaming\Microsoft\Teams
This folder is configured in our redirections.xml
The result was that the logon process tries to setup the redirections and fails at the above folder. The FSLogix profile is then unloaded and the user is unable to work as they have a blank profile.
It does seem quite severe that a corrupt folder can cause such disruption. It would be better if FSL allowed the redirections / logon to continue and loaded the profile but presented an error. Then the end-user would not face downtime.
See case Microsoft case 32794828 for details.