Issue while creating ad ma in MIM 4.6.673.0

Question

Hi Team,
I am in process to upgrade MIM from SP1 to SP2 and having a weird issue with one of the AD MAs.
Steps:
a. I've built a new Sync machine and installed SP1.
b. Pointed to Old FIMSync DB and applied Encrypt key.
c. Ran initial full import on all the MAs and completed successfully (23 MAs out of which 2 AD MAs.)
d. Applied SP2 hotfix (4.6.673.0) and rebooted the machine.
e. All the MAs are working, except external AD MA.
f. Keep getting 'no-start-connection' status. Error: 'Invalid Credentials', Error code: 0x31
g. But I can use the same service account to bind via LDP console.
h. Ran telnet and ping, the external domain is resolving.

Could you please guide me if I am missing anything?
Regards,
Sri