Turning off ACS mode is easy. Just shutdown Daemon Manager, and run:

NMSROOT/bin/perl NMSROOT/bin/ResetLoginModule.pl

The error you're getting, however, points to a problem with the ACS admin user you configured under your AAA settings in LMS. This user must have full access rights to ACS. The best way to see if this is the case is to go to Administration Control and click on the admin's username. On the next page, ALL boxes must be checked.

There is another caveat, though. If this is an ACS appliance, the appliance administrator cannot be the ACS administrator you specify in LMS. The appliance admin is special, and does not have CGI rights. You can see if the current admin user is the appliance user by going to Administration Control in ACS. The top line will say which user is the appliance admin.

If the current ACS admin user is the appliance admin, create a new admin user, grant that user al ACS rights, and update LMS with the new username and password.