[2021] Very complicated e-mail situation

Hello everyone,

I think the support had some trouble understanding me but I'm trapped in a very complicated situation with several e-mail addresses:

My own e-mail A is my Xbox account and according to my Microsoft settings, there's a connection to Uplay. But when I open at my Xbox the Ubisoft connect app, I can see the address B which is also my brothers Xbox login e-mail. I was also able to log in with address B to Ubisoft in my internet browser and tried to dissolve the situation by changing the address B to address C.

Now, I cannot login into the Ubisoft connect app on the Xbox (allegedly server problems) and there seems no possibility to gain access to the mysterious account with address B that is shown in the Xbox app but seemed to be a standalone Ubisoft account also.

I was talking to the German-speaking support desk but they said, it's on Microsoft's side but trying to dissolve the connection to Ubisoft in my Microsoft account (address A) didn't help.

Are there any experts around?