Never mind my first issue, I just solved it.
I deleted all entries (had only 3) in the following tables using "phpmyadmin":
After that the list with the files I already uploaded to i-doit was emtpy obviously. I uploaded a new file to my object (Router) and it works like it is supposed to now.
The thing I noticed though is that those 2 tables stay empty, although I uploaded that new file.