So it would be help a lot if you can find a simple pattern to provoke this error - as specific as possible.
Screen shots would help too, as described above it doesn't seems probably that the error is really location related. The Elen'lu Isles are generally not affected by this mod. Aman'lu is affected by the mod though but improbable that something critical is triggered when leaving this area again).
It's more likely that errors depend on any actions, however not every action instantly has an effect too, sometimes there is a delay between making the whole situation quite intransparent.

Alternativey, if you like to do your own multiplayer test with your friend, you both also could play with DungeonSiege2Mod.exe from the toolkit. There will be a new installations folder, but resources and mods can remain where they are (auto-detected), although savegames have to be copied manually to the new "Dungeon Siege 2 Mod" folder within "My Documents / My Games", and you should start the games with the loadanysave parameter.
Benefit of this approach is that on server and on the client machine there will be "Logs" folders with the errors-computername.log file that eventually lists critical errors.