I see a lot of that on our server.
My guess is, itÂ's that thingy that compares files on the clients in order to uncover cheaters. HavenÂ't looked, but I seem to recall that some file(s) are chosen at random and compared on all clients. So the error could occur when a client does not have an addon that is on the server, even though itÂ's NOT needed for the current mission or the client has that addon in a different folder. (OFP/addons or Res/addons)