Just before receiving your message I just removed the rule with:
sudo iptables -t nat -D PREROUTING 1
And that solved the issue! I’m extremely grateful for your help. I’m also curious whether there’s something that will re-emplement the rule on reboot, like you wondered. I’m going to try to reboot now and see what happens.
Edit: Even after rebooting, the site is still correctly served using HTTPS. Success!