Also happened to me, even on vShield Manager 5.1.2a (5.1.2-997359, with maintenance update kit)
Fixed just by reconnecting VC in vShileld Manager web UI (no other restarts).
The root cause of this error in my case was the fact that I updated SSL certificate of vCenter, and vShield Mgr stopped trusting it and lost connection to vCenter.
So when reconnecting i had to accept new fingerprint of SSL certificate of vCenter.