We just ran into an issue where users who log out of their phone then attempt to log back in get a "Cannot Display" error on the screen of their Mitel 6757i telephones. This may be cause by an automatic module update over the weekend - will post again if specific issue is found.

A rollback of these two modules seemed to resolve the issue for now:

Phone Apps 15.0.19.67 - upgrade available (15.0.19.81)
EndPoint Manager 15.0.41.25 - upgrade available (15.0.41.35)

Sangoma issued Rest Apps v15.0.19.83 which appears to be a proper resolution this issue.