I’ve been working extensively with Magento in the past few weeks and while it’s an amazing e-commerce system, their support and documentation is pretty terrible. A common error with the past few versions (I’m using 18.104.22.168) is that the page freezes with “Please Wait” showing when you try to add products to categories, or add/edit categories themselves. Some users have also reported the same issue when uploading images or adding/edit products themselves. These are some fixes that people have suggested. If you’re having trouble, give them a try and leave a comment if you want me to add a fix to the list.
1 . Try a different browser
This fixed the problem for me, as I was using Firefox 3.6 (OS X) to access the admin backend, and switching to Safari 5 (I’m guessing Safari 4 would work also) fixed the issue. I tested this on IE7 (WinXP) and it also worked.
2. Disable mod_security and add whitelist
Check your mod_security audit log and if you see entries from the admin backend, have your host investigate and temporarily disable the module. If your issues goes away, ask your host to whitelist your admin backend.
3. Check your www vs. no www redirects
Make sure you’re only using one form of redirection, either in cPanel, Magento, or .htaccess. See this post for in-depth instructions.
Hope these help!