Does your server host use UTime or one of it derivatives? (There were several due to its popularity)
Though removing files from your local client may fix it temporarily, as soon as you connect to a server running it, whatever is causing conflict on your machine (if it's running ok for others) will return as soon as you connect to the server again.
Try checking the local files on your machine.
<your_gmod_folder>/cache/dua <-- remove/delete all files (they get redownloaded when connecting to a server)
<your_gmod_folder>/addons/UTime <-- remove/backup to somewhere else
<your_gmod_folder>/lua/autorun <-- any files there in any folder named cl_utime.
Though that error is indeed client, if other players aren't getting it and they are running UTime fine, then something on your installation is causing it.
Most likely a conflicting addon, perhaps even an older version of ULib itself.
EDIT - Also, perhaps you didn't search our forum well enough.
I found this searching for "upvalue gpanel" -
http://forums.ulyssesmod.net/index.php/topic,4445.msg20278.html#msg20278Its a fix that would need to be done on any server running an old version if the fixes above for you didn't work.