Forms authentication not working properly in Teams SharePoint App

[Thomsch]

I've created a simple Teams SharePoint App via AppStudio.

At the SharePoint page, which is a Communication site, a couple of stream videos are included and also a Forms form. For a couple of users the forms form cannot load but looks like that:

Nothing changes if the users click on "Continue".

For some users the same error occurs with the Stream videos too.

The forms form is approved for all company users.

[#2]

@Thomsch - Did you add 

"supportedHosts": ["SharePointWebPart","TeamsTab","SharePointFullPage"] in the share point manifest? Could you please conform?

[#3]

@Nikitha-MSFT Can you please direct me to the place where to edit the SharePoint manifest? For most user this problem is not occuring. But a couple of them have this problem.

[#4]

@Thomsch Did you add the authentication to your forms? Could you please conform?