The app didn't start correctly. Try refreshing your browser
in a new created Sharepoint list (today) as well in one from the 19/2/20 (everything working fine at this date) the user is not able to open the Powerapps form (created from SP list) anymore. The error message is: "The app didn't start correctly. Try refreshing your browser."
The user who gets the error has site owner permission. I'm always able to open the form (as a creator and publisher of the form)
You can assign the Power Apps per app baseline access individually to users or assign the license to a security group that contains all users that will receive a Power Apps per app plan.
This step is optional, at this point the admin can turn off AllowAdHocSubscriptions for their tenant by using the following command: Set-MsolCompanySettings -AllowAdHocSubscriptions $false
So it seems to have been a licensing problem that the form could not be opened any more. After three days of observation the solution seems to be stable. At the end we switched the ad-hoc subscriptions off again 🙂