[syslog-ng] syslog-ng is ignoring a network source

Hello! I try to send syslogs from two firewalls to my syslog-ng host. The first one worked immediately, but logs of the second firewall seems to be ignored. Both syslogs come in via UDP on port 514 and I can see them in Wireshark on my syslog-ng host, but they disappear for my second firewall in a black hole. Both have Logstash as their destination configured. Any ideas? Thanks in advance! Cheers, Ben