TypeError: Cannot read property 'toLowerCase' of null

I have a strange issue on a test environment in the mendix cloud. When a user has a certain user role, the client won't startup at all. Instead I see a script Error:

TypeError: Cannot read property 'toLowerCase' of null

I've already pinpointed this to a call by mxui.js to

mx.session.getUserRoles("Name")> (9) [null, "UserRole", ....

So the user has the role but the role information is empty (the first one is null)

If I fetch is with a separate XPATH retrieve from the client, all data is there

//System.UserRole[System.UserRoles='[%CurrentUser%]']

Anybody encountered a similar issue yet?