I work on multiple computers and sometimes things don't go so smoothly as they should.
If you work with opencart and you are getting some undefined variable errors, it might be the case that vqmod is no generating the vqcache files as is should.
Also if you work with git and have an ignore for cache files, when you clone the repo you will end up with the cache folder empty. Sometimes a simple refresh won't build the cache files.
In this ocasions, try:
Stupid as it sounds, the latter solved my issue.