Works fine here...
Are there any other insert effects on the Omni channel strip? Click and hold on them and make sure they are in Stereo Mode.
Is the Level Meter display in the channel strip showing two bars (stereo) or only one (mono)?
What version of Logic are you running and is it in 32 or 64-bit mode?
No problem Soren!
btw, for Audio Channel Strips, you can additionally set the mono, stereo, left, right, surround format by click-holding on the format button under the level meter of the channel strip.