Please use the search box from front page to find any number of posts regarding search term "invalid lua", but basically 3 topics;
All of them relate to 1) download cache not having latest ULib and ULX SVN code, 2) not using SVN of both, and or 3 ) Gmod updates over the past 3 years that have broken our "release" version of ULib and ULX.
Though I didn't take long enough to read all the search listing results, I could also imagine another mod conflicting with ULib.
Where did you get the files, our downloads page, or SVN? If SVN, did you get both ULib and ULX?
If SVN, did you delete the old release code before checking out SVN?
See the other search results, many other fixes for your error, I believe all of which worked for one person or another.