Hi Thomas!
We've also had some difficulties with loading .osgb files even within one of the built-in sample projects. I don't remember which model was it, but we had a file which could be loaded on half of our Linux machines, and couldn't on the other, nor the Windows machines, etc. The key was that there are considerably different versions of pre-built OSG packages available on different OS-es / distros.
I think the error is because the OSGB format and its [de]serializers in OSG have changed from one version to another in a not-entirely-compatible way, and OSG being a community-maintained open-source project (AFAIK?), this might not have been handled in the most graceful way possible. I don't know for sure though...
So if you still have a version of OSG at hand that can load the model, or you have it in some other format, you could try converting/exporting it to a few different formats, and look at how well they work using different versions of OMNeT++/OSG.
If you have a simple static mesh with some colors/normals/UV maps/textures, I think you have a fair chance of getting at least a few to work, but with any more advanced stuff, like animations and object hierarchies, maybe not so much.
The osgconv utility can help you with this, also take a look at this table here:
http://trac.openscenegraph.org/projects/osg//wiki/Support/UserGuides/PluginsAgain, I have no idea how accurate and up-to-date it is.
I'm excited to see the awesome 3D visualizations you (and anyone else) will create using OSG in OMNeT++!
Attila