Unknown user on closing task (my own account!)

[Marcus Grondstein]

If I will close a task, my own current user will not be used. A error is shown. If I choose another user, it works. I am in the same group as other user?.

Is it possible that the problem is my privileges as organization admin?
I am a organization admin of another company too.

[Pavan]

@Marcus Grondstein  Please check your project permissions, by using permission helper.

[Marcus Grondstein]

Thanks for the tip but this does not solve the problem. The helper shows that I have permission for all selectable permission-items.
It is strange that I can change the person "zugewiesene Person" after I have saved the task with another one!!??
Currently I have to do this to close the task with me as related User (zugewiesene Person):
1. Start closing task
2. Change user to other than me
3. save closing
4. open closed task
5. change user to me (here it is possible!!??)