How many LEDs are on the board?
One (1): Status (Green) = CXM
Three (3): Status (Green) + Test (Yellow) + Fault (Red) = DXM
From the discussion so far, I get the impression that you have a single Status (Green) LED and thus an older CXM board.
I went back through the CXM manual and you have to put it in Test Mode by momentarily shorting the two TEST terminals to determine what is causing the fast flash indicating a lockout.
Test Mode - Test Mode allows the service personnel to check the operation of the control in a timely manner. By momentarily shorting the test terminals, the CXM control enters a 20 minute Test Mode period in which all time delays are sped up 15 times. Upon entering Test Mode, the Status LED will flash a code representing the last fault. For diagnostic ease at the thermostat, the Alarm Relay will also cycle during Test Mode. The Alarm Relay will cycle on and off similar to the status LED to indicate a code representing the last fault, at the thermostat.
Fault Retry - In Fault Retry Mode, the Status LED begins slow flashing to signal that the control is trying to recover from a fault input. The CXM control will stage off the outputs and then “try again” to satisfy the thermostat "Y" input call. Once the thermostat input calls are satisfied, the control will continue on as if no fault occurred. If 3 consecutive faults occur without satisfying the thermostat "Y" input call, then the control will go into Lockout Mode. The last fault causing the lockout will be stored in memory and can be viewed by going into Test Mode.
Test Mode can be exited by shorting the test terminals for 3 seconds.