while I am somehow always losing connection to my chat with Predrag, I thought it might be best to write it here down:
My receptionists (role based on the "author" role with some additional capabilities) cannot change the sidebars of posts. Although they have the possibility to choose a different sidebar in each post, it does not apply their changes after saving the post. The only person who seems to be able to change sidebars, is me.
I also use the plugin "User Role Editor" but I just don't find the possibility to assign this capability there.