Hmm... Although that does seem to change the result to the better, it doesn't seem to be the source of the problem. DASM still doesn't output any file, and by the looks of the message it doesn't even seem to read the file:
Maybe it's my version of DASM? All of the topics I've read have pointed me to a domain called http://atari2600.org/dasm, but this link is down and therefore I have no clue on what version I really should download. I'm really sorry for bothering you people so much, but making my own 2600 game has been a dream of mine for a long time. Thanks for understanding!