Hi Björn,

Excuse for delay with answer – I had 2 weeks vacation.
Did you apply Admin menu access restrictions for role of this user?
If you use “Block not selected” model for admin menu restriction, it may be a reason why user is redirected to to the dashboard when he try to open URL with additional parameter. Read carefully the final part of the documentation page.
I hope that switching to the “block selectes” model or tuning “URL arguments white list” will help you to resolve this issue.