Hi Sebastian,
The number of background genes does not matter but it has to include genes that you want to test.
If you didn't include the testing genes could you try including them?
If you still get the same error, could you share your GENE2FUNC job ID? I'll have a look from the server side.
PS. I'm sorry but the host server is currently down, I'm working on it but might take some time to be back online.
Best,
Kyoko