There are a couple of lines in the log file which suggest to me that GNOME thinks it is starting up on a 3D (i.e. GPU accelerated) X server:
The X server started by ThinLinc is not 3D however, so I suspect this is what is causing it to crash.
A common reason for GNOME thinking it is starting on a 3D X server, is that there is already one running as the same user. But since you’re not logged in anywhere else as this user, I don’t see how that would be the case. You could try checking the process list with ps if you wanted to make sure though.
Otherwise, it’s not clear to me what’s happening, I’m afraid. If you’re using an OEM version of Ubuntu (i.e. provided by your hardware vendor), then perhaps they are doing something non-standard with their desktop sessions, I’m not sure.
A completely different approach would be to use a different desktop environment altogether, something more lightweight such as MATE or XFCE. Since there are default profiles already configured for these in ThinLinc, it should be sufficient to just install the relevant packages for your distribution.
Hope that helps.