"Login error - There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Go back to the previous page, reload that page and then try again."
For the life of me, I do not remember exactly what I've done to my system which caused this error. Maybe I did change the user or group of my mediawiki directory (recursively).
Solution:
Change to the location of your wiki directory, e.g.:
# cd /var/www/html/wiki
Modify LocalSettings.php:
# vi LocalSettings.php
Add as the last line to the config file before "?>":
session_save_path("tmp");
Save the file.
Create a 'tmp' directory on the root of your wiki directory:
# mkdir tmp
Change directory permissions
# chmod 777 tmp
This should prevent the login error from showing up, at least in my case.