Error opening Add-in after installation in Teams Channel

[RobHagman]

After the installation on first launch/use we get the following error for some users, for other users it is working how ever:
(Translated from the original Dutch)
The request was not send or no answer received from the server, check your internet connection
Server Reply: Project User Settings failed: null

Probably a configuration issue at our end, but can you give a hint on what may be causing this,
we kind of stuck trying to solve this.