The motd is sent by code on the server.
You download to your cache folder the client side files.
You could try deleting your local gmod/cache folder, but, just remember, all the servers you join will require you to download all the lua files again (most other files (wav, materials, models, etc) won't have to be re-downloaded)
If you have a local copy of ULX on your machine, it _shouldn't_ make a difference.
(But, I guess it could)
Edit - Try deleting your local gmod/data/ulx folder too.. perhaps the motd file ULX writes there got locked/read-only/corrupt?