El Capitan Server - Mail is not Responding

Today I have installed the latest security update on my El Capitan Server (10.11.6) running Server 5.1.7 -

Unfortunately 😟 !

After that, the mail server does not answer any more. In the log files I do find a lot messages like:

... kernel[0]: Sandbox: clamd(339) deny(1) file-write-create /Library/Logs/Mail/clamav.log-20160911_130524

and my mail clients (either from OS X or IOS) complain, that they can not reach my mail server.

Does any one have a hint how to solve this - or shall I reinstall the version before the security update from backup ?

Thanks