Welcome to the forums.
Has this been a running installation that's gone wrong or is it a new install? What happened on your server around the time of the first error you posted above? Did you do any system update, did you have any system problems/hardware failure? Apart from what you've posted is there any errors in the /opt/zimbra/log/mailbox.log or /var/log/zimbra.log around that time?
Is this the sort of errors you're seeing over and over on your system? If it is, there is one quick check you can do for this problem. There's a package called 'gmp' that's required by the a/v stuff, look in your package manager and make sure it's installed. If it is installed the remove it and re-install it and see if that helps.
Jul 14 17:19:01 chinook postfix/smtp: C634A538104: to=<firstname.lastname@example.org>, relay=127.0.0.1[127.0.0.1], delay=10017, status=deferred (host 127.0.0.1[127.0.0.1] said: 451 4.5.0 Error in processing, id=10746-01, virus_scan FAILED: virus_scan: ALL VIRUS SCANNERS FAILED: ClamAV-clamd av-scanner FAILED: Too many retries to talk to 127.0.0.1:3310 (Can't connect to INET socket 127.0.0.1:3310: Connection refused) at (eval 54) line 269. (in reply to end of DATA command))
Jul 14 17:19:01 chinook amavis: (10741-02) (!!)ClamAV-clamd av-scanner FAILED: Too many retries to talk to 127.0.0.1:3310 (Can't connect to INET socket 127.0.0.1:3310: Connection refused) at (eval 54) line 269.
I'm a bit confused by your comment here: "I also tried to upgrade to the newest open source version but I'm unable to because i do not have a key file." - you don't need any 'key file' or licence for the OSS version. You have the OSS or NE version currently installed?