Can't change NIOSII version

I use Quartus 5.1 Web edition and Nios II 5.1 with a full license.

When I change the NIOS version from Standard to either Economy or Full, it won&#39;t compile any longer in Quartus and abort compilation with an error stating that it can&#39;t instantiate something in cpu_testbench.vhd http://forum.niosforum.com/work2/style_emoticons/<#EMO_DIR#>/sad.gif

I already tried removing and re-adding the CPU in SOPC builder but that didn&#39;t help. When I set it back to Standard everything is fine again.