Monitor instance bigip *.*.*.*:443 UP --> DOWN from gtmd (no reply from big3d: timed out)

Sounds like GTM is having issues talking to the big3d daemon on the LTM in one DC.

 

 

On that LTM, check whether big3d is running properly, "bigstart status big3d" and if necessary, try restarting the service (bigstart restart big3d)