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I was having this issue, after talking to support, here are the solutions:

If the only admin is locked out, there is no other choice than the following:
1)Log in to the Control Server
2)Stop the ScreenConnect Session Manager service (This will stop the Web Server and Relay service as well)
3)Navigate to C:\Program Files (x86)\ScreenConnect\App_Data
4)Edit User.xml
5)Edit line to 'false'
6)Edit line to '0'
7)Start ScreenConnect Services
8)Attempt to login again

If multiple admins are available, Login as another admin and reset the password of the user that is locked out, which will reset the flag IsLockedOut to 'False' automatically. (administrator > security > show user table > edit user)

I hope this help.