Understood. Another user had the idea a while back about launching ThinLinc directly from GDM as a single application, rather than starting an entire desktop environment. I haven’t had a chance to test whether this might be a solution or not, but it seems like GDM might intervene regardless of the session type anyway?
If so, the only solution I can think of would be to log in from another device and end the ThinLinc session gracefully. I actually wasn’t aware that GDM is so strict regarding multiple sessions; most of the issues I’ve seen occur with multiple sessions of the same type only. I might have to do some more investigation here.
Thanks for the report!