I may have found my own answer when I
finally got to see the session detail below. It led me to a post
describing how the session data on a server can get corrupted.
My only question: how can 14 servers in different locations get
corrupted all at the same time? Here's what I've run to fix 2
of the 14 servers so far:
$ sudo rm -f /var/lib/x2go/x2go_sessions
$ sudo x2godbadmin --createdb
Hopefully, I won't need to post about this issue again and maybe
this information will be helpful to someone else.