OpenVPN daemon constantly outputs error: "Bad argument `MASQUERADE'"

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I’ve been trying to install OpenVPN in my server, specifically in order to use Pi-Hole on my personal devices as explicitly recommended by the Pi-Hole developers. Unfortunately, it seems like the package has been unmaintained for about a year, and since then the iptables syntax seems to have varied, which causes the daemon to constantly send mails to the administrator with the following error:

Bad argument `MASQUERADE'
Try `iptables -h' or 'iptables --help' for more information.

I reported the bug already, hoping that someone knows how to properly configure iptables with the new arguments, but if someone can explain how to make the iptables rule to work again, I think I could submit a pull request with the required patch.