a specific user can't mount usb sticks

I have the problem that a specific user can't use usb sticks any more. they are not mounted automatically any more.
For some weird reason I changed the numeric userid and groupid of this user (actually to solve some problems with a NAS).

I saved the home directory of the user in question and deleted the user.
When I create a new user with the same numeric uid but a new username, it works.
When I create a new user with the same numeric and username, it doesn't work ...

using Ubuntu 14.04

thankful for any hint!