Warning: is_writable() [function.is-writable]: open_basedir restriction in effect. File(/var/lib/php5) is not within the allowed path(s):
was there no path output after the error message? could it be a change on your web host setup?
Sorry. In index.php, change
if(!<a class='mention' href='https://github.com/is_writable'>@is_writable</a>(ini_get('session.save_path')))
fixed in 
Just a follow-up note that there's no change to Piwik requirements. I checked the PHP bug tracker, and a bug in is_writable() behaviour wrt open_basedir() where is_writable() would return true, was fixed for php 5.1.3. This is already the minimum required version for Piwik.