Please provide a smartphone (etc) nslookup over an alternate wan connection to verify your ddns record is uptodate available ( full debugging output of your ddns daemon sanitized would also be useful )
@vgaetera, thank you for your solution provided link (Limit BusyBox symlinks/binaries to /bin and /sbin).
In the meantime, I reinstalled some packages (dnsmasq-full, firewall, adblock,...) and installed odhcpd (ipv4 version), additionally, which was installed as ipv6-only.
Restarted the router and everything works fine!
I've read the posts in the link above and it seems, the busybox update overwrote some packages and this might cause the issues.