[Mashiro]

I see nothing wrong so far (excluding the failed exim4 startup as it is unrelated to the issue in this thread). The rule set looks pretty much 100% like a generic default ufw rule set. Line 2 is the first rule and nothing wrong about it.

I'm not a ufw user and not a big fan of it either.

Here is another thing that might help. Open the file "/etc/default/ufw" and set "IPV6=yes" to "IPV6=no". If "IPV6=" is missing in the file just add "IPV6=no" yourself. After that reboot and see how it goes.