You're right. It seems that the problem is not related with ESXi. When I checked the logs, I saw that the proxy server tries to connect our old sametime server.
Bu I couldn't find where it's getting this connection information. I installed completely new sametime server and used everywhere the connection information of this new server.
Is it possible that there is still some connection information on the domino directory or LDAP configuration to get this old server connection information?