This is contradictory with the last statement made by LE staff in your old thread.
- There is no known connectivity problem between LE's secondary validation servers (AWS) and Hetzner
- I host several sites on Hetzner myself, without any issue
- Hetzner is not known to block IP addresses not requested by the customer
I strongly suggest you inspect your own firewall again*. The secondary validation IP addresses change frequently, which is likely why It worked for a while (and will likely continue working again in the future, until it breaks again).
Other than that, without more info from your side there is nothing this community can do for you.
*You can try with your iptables rules deleted:
# sudo iptables -F
will temporarily clear everything (until reboot/reload/reapply).