Hey Jens Bruckmann,
Does it show an error when you select the inner join option?
There are some great insight here, you might find a workaround here:
Let me know if it helped!
Also, sample data sources might be helpful here.
the issue was solved very simple. If you do a join with more than one WDC to an excel you have to do a left outer join to all files and not only to one. This sorted the Problem