we recently begun working on a web shop whose owner wanted color swatch option for her store. We felt very lucky that Magento CE 1.9.1 came with that functionality. So we upgraded store from 220.127.116.11 to 18.104.22.168 But as usual, Magento upgrade didn’t go without problems. After the upgrading was finish, we coldn’t open the store. All we saw was a blank screen with 500 Internal server error displayed.
We tried making a downgrade back to 22.214.171.124. and then again upgrade to 126.96.36.199 As soon as we downgraded the store it was working normally. But the minute we upgraded it, it again displayed error 500. This only ment that there was something wrong with Magento installation. After looking at the server logs, we found out the problem – permissions on index.php. The permissions on index.php were set to 664 meaning the group was able to write to the file. The minute we changed the permissions on index.php to 644 the store was working again.
Hope this helps 🙂