Hi,
Do you have phpMyAdmin installed on your server?
If yes, could you run this query on your phpMyAdmin?
REPLACE INTO `cms_modx`.modx_active_users (internalKey, username, lasthit, action, id, ip) values('1', 'admin', '1136560036', '', '', '127.0.0.1')
That code will insert manager info into the database, and see if it returns any error or not.
I don’t have MySql 5 with me, so if you would like to go through with this step-by-step with us, it will help us a lot as a developer of MODx.
Report any error when you run those query.
Note: It might be an incompatibility issue with the ommited syntax from the previous version, which is common. I believe Joomla and the others didn’t use that syntax, and we accidentally is using those syntax, either during installation process or inside our system.