After I found some more time to debug I think I fixed the problem.
Somehow this elf warning was triggered by some iptables modules that where not loaded automatically.
Basically I had the same error as described here
mailcow/mailcow-dockerized4802
The fix was to insmod the missing modules manually.
modprobe -v ip6table_filter
modprobe -v iptable_filter
docker-compose restart netfilter-mailcow
and it works as expected.
netfilter-mailcow-1 | Using IPTables backend
netfilter-mailcow-1 | Clearing all bans
netfilter-mailcow-1 | Initializing mailcow netfilter chain
netfilter-mailcow-1 | Watching Redis channel F2B_CHANNEL