Thanks, as I said, I had already tried the uninstall route, including AIR and also clearing all the system files.
So I disabled Avast and lo and behold this worked. But naturally I don't want Avast disabled.
So I re-enabled Avast and tried to open a gallery and this time Avast popped up a warning that JuiceboxPro was attempting to access protected files. It also gave me the option of allowing the app. Which I now have done.
Strange thing is... why didn't it ask me before. It only asked after I disabled and re-enabled.
One for your knowledgebase I guess.
Thanks for your help.