I can't add host to vCenter?!

I have very strange problem from today.

We have 2 servers (A and B), SAS shared storage and Linux vCenter. Licence is Essential Plus.

Everything was fine untill this morning - I couldn't connect to vCenter with vSphere client. Sometimes connection establish for a few seconds and then interupt, usually with message: VPXClient has stoped working.

I tried everything, even to deploy new vCenter, and I realize that connection stoped when I try to add serverA?!

IP addressesing is ok, storage is ok, everything is ok but I cannot add server to vCenter.

Please help.

Thanks