RT 2.1.33 strange URL generation

I managed to install RT 2.1.33

Now I encountered a new problem:

RT is somtimes adding the actual path to the url after the host.

Examples:

http://hostname/rt3 Search links to
http://hostname/rt3/rt3/Search/Listing.html
http://hostname/rt3/Admin/ Groups in the small menu links to
http://hostname/rt3/Admin/rt3/Admin/Groups/
The link below the horizontal line is correct
(http://hostname/rt3/Admin/Groups/)
http://hostname/rt3/Admin/Global/ Groups links to
http://hostname/rt3/Admin/Global/rt3/Admin/Groups/

any hints?

MfG
Andreas Banze