I know this is an old post...I just ran across this problem myself. I would attempt to login, end up on the login page again with no error. I tried the password reset functionality and continued to get the error message regarding the key being incorrect.
My solution was to do a repair on the MySQL database. The modx_session table had some corruption, several rows were purged during the repair. Once the repair was successful, I logged in everything worked fine.
I am on MODX Revolution 2.5.0-pl with MySQL 5.5.52.
Hope this information can help someone else!
