That's.. very strange-- The problem is being caused by this line here:
lua_temp\ulx\modules\cl\xgui_helpers.lua:354: attempt to call method 'LabelAsPecentage' (a nil value)
My first thought is that Garry spelled "Percentage" wrong, so he might have fixed it, but I just tried updating my ded server and everything, and it still works fine. Can you go to your server and open garrysmod/lua/menu/progressbar.lua, then look at lines 32-35? They should look like this:
function PANEL:LabelAsPecentage()
self.m_bLabelAsPercentage = true
self:UpdateText()
end
If the code is different, then that file has probably been modified and should be reverted back to it's old state (unless there was a good reason for it to be changed). If the file looks fine, then possibly another mod is conflicting with the progress bar? Can you think of any mods you have installed that use this progressbar?