Hello Arne,
This is probably related to the fact that you currently have a userrole that is not allowed to manage (see) some specific roles.
This can be configured under Security > User Roles > Role x > User Management at the bottom.
A nice tool to help you figure out permission issues in the future, would be to use the Bizzomate Dev Tools. I can highly recommend this tool for all Mendix developers.
Hi Arne,
There is an XPath constraint on which userroles a user is allowed to see.
[System.UserRoles = '[%CurrentUser%]' or System.grantableRoles[reversed()]/System.UserRole/System.UserRoles = '[%CurrentUser%]']
So a user should either have that role themselves, or have management access of that role (the checkboxes on the bottom of the user role popup in project Security).
Hope that helps!