Transpose Table: Required Additional Login Leads To An Error

Hi all,

I have synchronized Sharepoint data source with Power BI Desktop. Full access to the source has been granted. Sharepoint lists can be merged, but not transposed. System asks for an additional login, which happens via Microsoft Account. After logging in it starts synchronizing but then it shows an error (pic below).  Most of the suggested solutions from this forum already have been tried out. Hopefully one of you has a good idea on how to tackle it. In advance, thanks a lot!


Take care!