One of the buttons on my website does not respond properly and I cannot understand why...At first it works but then it does not.

Please go to http://www.artdaemon.com/early_works3.php click on the first image...popup shows...click on the right arrow...next page shows(this is the problematic page) ...Now click on the left arrow..works...now click on the right arrow..click again...third page appears... now try to go back to see page 1. clicking on the left arrow...You cannot!

Another weird thing is you click on the image 2. and a popup shows ( as it should but left arrow link which should take you to image 1. will not work.

Any ideas why this is happening?
Sinisa
artdaemon.com

P.S. io tried number of things including erasing the whole website , remaking the same page , renaming...etc no success!