It got it to work, xenial was broken, and everything prior had wrong
library version for the Mint 18.3 X-server. Since I am doing some work on the machine tonight I really needed it to work, it was either that or vnc which is extremely painful.
But you said the Xenial repo still fails to upgrade on Mint 18.3? Is that still the case?
I guess I'll just try to see what happens myself.
Mihai