Comment 26 for bug 1847275

Ran into the same problem. The service never recovers after this error, because the package is not shipped with a systemd unit, but with an old /etc/init.d/stunnel4 script.

I ended up uninstalling the Ubuntu version, compling 5.60 (latest at the time) from source and then running it with a makeshift systemd service (available at https://gist.github.com/Jalle19/60532513bbcec2fa630a9d8b576d91e3).