Could you check something on your server?
In /gamemodes/terrortown/gamemode/vgui/ there should be a "ProgressBar.lua" file. If it's not there, that could be the problem, but that's unlikely. Anyway, if you open the file and scroll all the way down, at the bottom should be a line like:
vgui.Register( "TTTProgressBar", PANEL, "DPanel" )
Is it different in your case?
I'm asking because the error happens when it fails to create the progress bar interface element, which should not be possible if the above stuff is correct