No, I did not have a local MATE session running.
I’ve had this problem with Ubuntu MATE 16.04, 18.04 and 20.04, using various different versions of ThinLinc. I just never took the time to investigate it before. It seems to be related to a similar bug in TightVNC and some other flavors of VNC. I believe ThinLinc uses some sort of VNC in their implementation.
Doing a search on the internet for the error message I posted yielded several pages, all with similar suggestions to fix it. It was primarily a matter of locating the correct file to modify.
It’d be nice if ThinLinc would find a way to address this without us having to modify our guest systems; but I’m just happy to have ThinLinc for free limited use !!!
Have a great day!