I'm encountering this error as well, but it doesn't appear to be very widespread. I've tested a wide range of airplanes from various developers at the same parking spot one after another, and the only one that causes this couatl crash is the Milviz 737. This is a new plane for me (I was living under a rock and didn't find out they were giving away all their P3D planes until a couple of days ago), so I don't know if GSX is supposed to work with this or worked with it in the past, but as of now it is throwing the same error message that others are reporting. Here's what I've noticed:
1) It only affects this one plane. Everything else in my collection seems to work normally.
2) It only affects passengers. If I use the cargo variant of the Milviz plane, cargo will unload normally. The moment I try to deboard crew or passengers from any of the variants, it immediately crashes couatl.
3) Changing settings through GSX config and manual edits to the [exits] section of the aircraft.cfg file don't seem to have any effect.
It must be some sort of problem with the exit configuration in this plane. Is there any way to work around this, or are some airplanes simply incompatible with GSX? Milviz isn't offering support anymore, which stands to reason given that they're giving their P3D planes away for nothing.
Suggestions welcome - it would be really nice to have a working combi plane. Error log from recent test attached in case it's of any use - mine seems a lot briefer than the others posted here, but the error message at the end is consistent.