"Sorry something went wrong"

Hello,

Recently, one of our customers is unable to access two different Forms.

For both of them he receives the error message:
"Sorry, something went wrong.
Please make sure you have permission to access this form."

However, in the admin panel I was able to confirm that said customer has the license for this app.

Unfortunately, the following tutorial did not help either:
https://docs.microsoft.com/en-us/sharepoint/troubleshoot/administration/microsoft-forms-shows-sorry-something-went-wrong

Does anyone have an idea what else could be the problem?

I attached a screenshot of said error message.

Many thanks in advance