I couln't update my joomla site from 3.3.3 to 3.3.6. I got an ajax error.
Error log reported
End of script output before headers: restore.php
In other cases this issue could be caused from a permission issue
CHMOD
find ./ -type f -exec chmod 664 {} \;
find ./ -type d -exec chmod 755 {} \;
In my case the issue was caused from a php package: php5-xcache.
I just removed the package, restarted apache, updated joomla, installed xcache again and restarted apache
REMOVE XCACHE
apt-get remove php5-xcache
INSTALL XCACHE
apt-get install php5-xcache
