In further grepping, I discovered that the errors I mentioned:

Error: Failure negotiating the session in stage '7'. Error: Wrong version or invalid session authentication cookie.

are listed in the /client-side/ session logs:

=== $ cat ~/.x2go/S-hmangala-55-1381535091_stRgnome-terminal_dp24/sessions Loop: PANIC! The remote NX proxy closed the connection. Loop: PANIC! Failure negotiating the session in stage '7'. Loop: PANIC! Wrong version or invalid session authentication cookie.

but those strings are not found in the client, nor in the server-side nxagent shell script so .. where are they coming from..?

This has been noted in other list postings: <https://bbs.archlinux.org/viewtopic.php?id=144821>

Harry Mangalam - Research Computing, OIT, Rm 225 MSTB, UC Irvine [m/c 2225] / 92697 Google Voice Multiplexer: (949) 478-4487 415 South Circle View Dr, Irvine, CA, 92697 [shipping] MSTB Lat/Long: (33.642025,-117.844414) (paste into Google Maps)