Certificate Renewal Fail / Reboot

I struggled with certificate renewal. It said connection problems, but my ports were open (80/443). I found another thread, where deleting the IP6 DNS entry was a solution, so I tried this. After a few minutes, certbot --config-dir /config-dir renew ran successfully (wasn’t, before). What could be the root cause of this problem?

Also, after successful renewal, I had to reboot reboot as root was ok, but I wonder what the recommended reboot command is, gracefully stopping docker, etc?

Here it says, the IPv6 problem should be resolved: Certificate renewal fails, UI also fails

Pointers welcome.