I've personally not seen this error while running ULX, though I have seen it while attempting to join a friends non-ulx server.
It was after his server had been running for several hours, and many players had various builds/projects about the map. A Snapshot is the 'image' of various pieces/placements/player locations/etc the server sends to your client. (Some may not have known this, if you did, my apologies)
I presume you're getting this upon server start.
1) Can you run other maps and not get this error? (Some maps cause this easier than others, freespace03 I know of seems to)
2) Does this only occur when you have eSwep loaded? (You mentioned it as though you just added and it broke)
3) Have you recently added any other mods? (To server, don't have to be lua plugins)
4) Tried running ULX only? Its possible one of the weapons (if you have any) is causing this error
If this just started, I'd guestimate(tm) that either eSwep has some sort of issue, or one of your sweps does.