The session log file is located in /var/opt/thinlinc/sessions//last/xinit.log, and will usually contain valuable information for debugging session start up problems.
Sometimes it might also be worth to take a look in the client log file, which is located in %TMP%/tlclient.log on Microsoft Windows and ~/.thinlinc/tlclient.log on Linux/macOS.
Since you recently upgraded your ThinLinc server I would suggest you to start look at the xinit.log.
You’re more than welcome to share the log if you don’t feel comfortable in debugging it yourself.
It was exactly that issue, purging “upstart” has fixed my problem.
Not sure why it was present, as the machine started out as 16.04, and the ticket suggested that upstart should only have been present if the machine was originally 14.04 or older, but at least it is fixed.