Cannot refresh Dataset after UPN change

We are in a process of Office 365 migration of our user accounts. User
[email protected] is also an email user, but has full rights to PowerBI, he creates PowerBI dashboards.

I changed his UPN from [email protected] to [email protected] but afterwards he was not able to manage his own PowerBI dashboards.

So I changed back his UPN to [email protected], this restored parts of the PowerBI dashboard functionality, BUT user is now unable to change credentials of PowerBI gateways. Also the PowerBI gateways are greyed out and we cannot refresh data.

Any ideas how to fix this? It looks to me as a fault at the Azure AD side wich I cannot correct myself.