Bad Gateway when adding certain users to channel

I have a very interesting problem where i get a 502 Bad Gateway error when adding certain users to a team or channel via the graph API.

https://docs.microsoft.com/en-us/graph/api/team-post-members?view=graph-rest-beta&tabs=http

https://docs.microsoft.com/en-us/graph/api/conversationmember-add?view=graph-rest-beta&tabs=http

Both fail for these users. The users are always the same. 

If use the group to add a member to the team

https://docs.microsoft.com/en-us/graph/api/group-post-members?view=graph-rest-beta&tabs=http

it works for these users. 

But as far as i know there is no equivalent way to add members to channels. 

I couldn't find any obvious difference between the users. If i add them manually via the Teams client there are no problems.

Any one got an idea whats the problem or how to solve this?

Thanks