Mercy, Cornflake, you're both running Listen servers, right?
That might be a commonality. I have no idea from there though.
While trying to work/reproduce the error people are getting with ULib/ULX - invalid Lua, I noticed in a single player game (didn't check listen or ded) that an old version (didn't update it to test either) of Pheonix mods was trying to read data from data\..\spawnlists\<files> ... Gmod was saying 'sorry, can't do that'
Though the error you're getting is from files that are IN data\ and aren't trying to navigate out, perhaps it's a listen server issue.