Option "source_ips" not working

[northboot]

When using the option source_ips in settings.yml, the instance isn't able to perform any search requests anymore, regardless of the specified search engine, it just shows an error message that there was an error receiving answers from the search engines. This happens no matter if

1. only the primary IP address is specified in source_ips
2. only the secondary IP address is specified in source_ips
3. both IPs are specified in source_ips

Both addresses are public IPs.