SEP11 clients on Win2000 not upgrading from SEP12.1 server

I am running two SEP 12.1.4023.4080 servers, one with internet access, another one with daily sync with the first for definitions upgrade.

On the second server I have ~ 10 clients with Windows 2000 and SEP 11.0.6300.803 installed. Since 24.06.2014 these clients stopped upgrading definitions. All other clients connected to this server are up to date (version 12.1.2015). The sync works great every night.

My system with SEP12 server and SEP11 clients on Windows 2000 worked with no problems until 24th of June. What can I do next to get the clients up to date?

I have allready try to restart the server and the client. No result.

Thank you!