We could not ping one of our servers. I looked at the event logs and found this entry:
Description: The IPSec driver has entered Block mode. IPSec will discard all inbound and outbound TCP/IP network traffic that is not permitted by boot-time IPSec Policy exemptions.
Really strange! I did not enable any IPsec policies, the firewall was also disabled on this server. So how did this happen?
Well, the server ran out of space and somehow a registry entry got corrupt.
Take a look at this KB article. The fix worked for us. Actually the whole key was missing.
When I registered the DLL , it re-created the key. Reboot and the problem is solved!