On refreshing the page - I got the error again! So much for the product categories module being disabled fixing the problem
thanks fcoulter, but the forum post you link to is about fixing the problem where it appears in the backend.
I've spent the whole day reading (among other threads on other forums) all 15 pages of this thread on the VM site and some users mention having (and some finding fixes for) the problem in the frontend too.
I've tried every fix I can find, including disabling every module, until I just had the virtuemart component, the main nav menu and the JA purity template. STILL THE SAME PROBLEM!
The only other suggestion I've found involves using a more up to date version of mootools for the front-end but leaving the existing one for the backend. It just looks really complex and difficult to maintain because changes may need making to source files with the install of every new module.
I'm now contemplating whether it would be better to use another e-commerce solution but this is a last resort. Sample shops linked to from Virtuemart's site have it all working fine (including using slimbox for larger views of images, and the categories module).
It sounds near impossible to find a "works for all" solution because every user will have different combinations of modules, templates etc, hence some of the solutions posted working some of the time for some of the users.