It looks like that I can't log in to my Joomla back end anymore. No error messages are shown, the log in screen just stays there. If I enter a wrong password on purpose, I get a warning that the username and password doesn't match.
All I did was that I made a backup of /components/com_users directory with a different name and deleted the original. I noticed that I made an error since I couldn't log in anymore and copied the directory back with the original com_users name. After this I couldn't log in either.
How do I fix my mistake?