Hi Fabian,

Sorry for the late answer. You're assumption that the AssignedUsers value is the cause of the error is right. The AssignedUsers expects an array of usernames, instead of user guids.

I tested the change assignment call and it works (given that all parameters are correct). As a general note though, if something goes wrong, the first thing to check is the Application Service log, located under C:\%programdata%\SDL\Service\Logs\Sdl.Application.log. In this case you should have an error similar to:
2015-10-02 13:53:28.759#Sdl.StudioServer.Services.ProjectServer.RestApi.ExceptionLoggerFilter#web service error
System.InvalidOperationException: User 'ab651f46-41dd-4b47-9933-04b645be8ad4' does not exist.
at Sdl.StudioServer.Services.ProjectServer.ProjectServerManager.CheckIfUserIsValid(String username)

I will make sure that we revise the documentation and improve on it for the upcoming releases. We will try to add more details on the expected parameters and more meaningful examples.

Thanks.