I'm experiencing the same issue again. Just updated to eCart610.
And just like two years ago (read above), "Add To Cart Button" is clickable, yet not responding at all.
You solved the problem beautifully last time, but it seems I lost the note I made about what you did in which file, and I can't remember what you did...
My OS is 10.9.5 in Japanese.
Did all the possible solutions (deleting .dat file, renaming configuration folder and re-installing eCart, updating all the other plugins) and nothing made a change.
Do I need a support ticket for this again?