No communication between SOA and clients

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We run Endpoint Protection Suite on a windows 2008 domain. The SOA is 1.3.3.89 and the avast Antivirus is 8.0.1606. Since mid of march the SOA is not detecting any of the clients on the network. The 3 servers running Windows 2008 are detected without problems.
Any advice?

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Today all clients are detected again by the SOA.
I have only worked on all the clients.
After the last update of the Antivirus to 8.0.1606 via the SOA-console, on all Windows 7 clients the WAC showed a warning, that no Antivirus was detected.
I started every client in safe-mode and remove Avast using the avastclear-program. After a restart, I reinstalled the Avast-Antivirus via the SOA-console.
I donot understand, why the clients are detected again by the SOA now.