SuperOffice / Sharepoint using internationam M365 Tenants

Dear all,

One of our customers has several international branches. Two of these countries work with SuperOffice using the same tenant.
We are currently trying to move the documents to Sharepoint but are having problems with our colleagues from abroad.
In the Microsoft world we have different tenants per country. The authorizations are set so that the AT colleagues can see and edit the documents on the GE Sharepoint.

Unfortunately, SuperOffice does not seem to support this, ALL users with user plan must be in the same Microsoft Entra subscription.

We made some tests to see what happens.
Transfer to sharepoint worked fine. The AT users can open and edit the documents from SO. Our only problem is that the AT users cannot create new documents. They get a 400 error.

Has anyone had this problem and solved it?

Thanks in advanced
Georg