Unbelievable that such a simple thing as an upgrade to a newer version of CMS may cause any headache these times. Recently I have received a warning message from my host (lunarpages.com) that I still use an old version (1.7.2) of Joomla! for my company site, while a newer version is available. I have tried to follow official upgrade procedures for about an hour without success, resulting in wrong layouts and half-working admin functionality, and finally ended up just downloading the complete package for Joomla_3.1.5-Stable-Full_Package.zip and simply unzipping it to the current installation folder. That did the trick (well, almost) leaving me with the similarly popular warning message
Your host needs to disable magic_quotes_gpc to run this version of Joomla!
After some googling around the following solution worked. I have created two files as shown below in the folder with Joomla! installation (so not affecting other websites) and that did the trick.
magic_quotes_gpc = Off
session.save_path = "/home/<user>/tmp"