Marathon NameError: default

I want to use Marathon for testing a GUI and we had it working before. Now the test cases have changed, so I had to modularize the whole Marathon project into smaller ones. I created the new projects out of Marathon and I got the usual folder structure (Fixtures, Modules, TestCases) and the .project configuration file. I let Marathon create my default Fixture and its contents are just the same as the one before. When I create a new Test script out of Marathon (or from eclipse, or from a txt-tool) and I try to run it or to record with Marathon, I always get an Python error:

ScriptException: NameError: default

I use the standard method at the beginning of my test scripts: useFixture(default), and even if I let Marathon create it automatically, it still cannot recognize 'default', although it asks me to overwrite the existing file default.py, which tells me, the marathon.fixture.dir property is correctly set to %marathon.project.dir%/Fixtures.

I have absolutely no idea what is wrong here and if I can't fix it, I wont be able to use Marathon for any more testing.

Somebody has an idea?

Thx, Sergio