This came to me as a surprise when I upgraded to vCenter Operations 5.0.3( vApp ) to vCenter Operations 5.6, instead of what I was expecting I got an empty screen when I started the vCenter Operations Manager website and the Dashboard. This left me confuced I started troubleshooting then to my good luck after some googling I found this thread on the on the vmware community.
http://communities.vmware.com/message/2158059 these guys helped a lot, but when you are free from the trouble it actually is a very easy fix,all you nedd to do is just clear your browser cache and then the dashboard will work well again.