Also would be good if you have a look to the latest vCenter 5.1 U2 > once problem was found 
.
now you have
vCenter Server™ 5.1.0a | 25. Okt. 2012 Build 880471
Latest ones:
vCenter Server 5.1 Update 2 | 16. Januar 2014 | Build 1473063
Best regards
max