I ran across another problem tonight with my Magneto installation.  For no good apparent reason my Add to Cart and Image Zoom suddenly stopped working.  I used the Firefox Error Console and this was my full error message:

Error: uncaught exception: [Exception... "Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE)"  nsresult: "0x80070057 (NS_ERROR_ILLEGAL_VALUE)"  location: "JS frame :: http://www.yoursite.com/magento/js/index.php?c=auto&f=,prototype/prototype.js,prototype/validation.js,scriptaculous/builder.js,scriptaculous/effects.js,scriptaculous/dragdrop.js,scriptaculous/controls.js,scriptaculous/slider.js,varien/js.js,varien/form.js,varien/menu.js,mage/translate.js,mage/cookies.js,varien/product.js,calendar/calendar.js,calendar/calendar-setup.js :: anonymous :: line 1528"  data: no]

After a lot of searching….well, I found nothing.  All the answers on the Magneto forum were no use to me at all.  After about 30 minutes of searching though, I did find an answer that works (at least for now).  I had to re-install the Lib_Js_Prototype using the Magnento Connect Manager.

Here is how you do that from the Magneto admin backend:

1) System – Magento Connect – Magento Connect Manager

2) You will be prompted for your admin user name and password

3) Select Lib_Js_Prototype and then choose from the drop down action menu Re-Install.

4) Click on Commit Changes

If everything goes ok, you should see something like this:

This seemed to solve the problem for me….at least for now.  Hopefully this works for you or at least helps you out somehow.