Microsoft has answered this:
Turn on, at least temporarily, ad-hoc subscriptions. Run the following PowerShell command:
Set-MsolCompanySettings -AllowAdHocSubscriptions $true
Add to your tenant the required ad-hoc subscription to use a Power Apps per app plan by selecting the following link and completing the sign-up process: https://signup.microsoft.com/signup?sku=bf666882-9c9b-4b2e-aa2f-4789b0a52ba2.
You are now able to assign the Power Apps per app baseline access license to all users expected to use a Power Apps per app plan. Assigning the baseline access plan can be done in https://portal.azure.com or https://admin.microsoft.com.
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 🙂
