In the meantime I tested some things in BIAB 2015, and RB as well.

To Percy,

I have followed your suggestions. So I loaded the Midi File and then with SEQ I removed track 10. Then I chose a new style. Btw, I presume that by choosing a new style BIAB comes out of what you called the SEQ mode, right? Then I saved the file in BIAB. And yes, you are correct, now I could add a RD to the song, so for that part it worked! Thanks for the help with that! But then when I played the song I noticed that several chords in the melody really didn’t match with the style instruments sound and sounded semi false.
My tuning settings are on default. Do you have any idea how this can happen?

To Noel,

You suggested to use RB instead of BIAB for Midi files, so I tried this in RB as well. I know this is the BIAB forum so I will keep it short here. I loaded the Midi file into RB and then erased track 10, then I could successfully load a RD into that track and RB reported the track as Audio track. On itself it played well, though because I wanted to test with another RD I wanted to delete the RD from the track again. Then I got an access violation error in Realband.exe which I already reported to PGMusic.

greetings,

Valentino