Do you have somewhere to post one of your failing SEQ files?
On the target tracks; is the Track Type MIDI or Audio?
IOW, what is your default track type? I assume Audio since you said you rarely use MIDI (which seems weird for a guy with a bunch of synths).
If Audio, are they MONO or Stereo?

Just pondering possible causes since it seems to be related to the file you have saved.
.. or your Untitled.Seq file if you haven't ever saved it.

Wonder if renaming that file so it creates a new one would do anything.
I'm pretty sure RB opens that file (if it exists) when you open RB, as a default.
If that default file it opens is corrupted, I suppose everything you did after that would have issues.

No harm trying .. force RB to create a brand new starting file (Untitled.SEQ) by renaming the old one .. in whatever folder you are running RB from.

At least then we know the base seq file template is clean
maybe??
Sure is a weird one from what we know so far



Make your sound your own!
.. I do not work here, but the benefits are still awesome