Sharepoint connector not working ("The connection xxx could not be found")

Hello,

Right now I am trying to connect our online sharpoint to M-Files. I want to sycronize documents from a Teams/sharepoint site collection with M-Files. So users can see those documents in M-files too.

While I was creating the externa Sharpoint connection I wasn't able to authenticate the index user or permission retriever. Both requests resulted in the error "Not found. (The connection xxxx could not be found.) 

Does anyone know what the problem is and how I can solve it?