hello orinoco,
I suspect you may have old mountpoints in the /media directory that have wrong owner + group (root root).
I suggest you remove them.
Before i plug in my USB stick my /media is empty:
musca@sidbox:~$ ls -l /media
total 0
After plugging in and selecting open with filemanager there is a mountpoint that belongs to the user musca:
musca@sidbox:~$ ls -l /media
total 8
drwx------ 7 musca musca 8192 Jan 1 1970 Spaceloop
After removing from "device notifier" /media is clean again:
musca@sidbox:~$ ls -l /media
total 0
Authorization is done via policykit-1, and the package usbmount is not needed (i removed it recently).
Please check theese packages:
chilan@sidbox:~$ dpkg -l | grep -e 'policykit-1' -e 'usbmount'
ii policykit-1 0.105-5 amd64 framework for managing administrative policies and privileges
rc usbmount 0.0.22 all automatically mount and unmount USB mass storage devices
greetings
musca