The FlowVPN client is free (and works with any provider).
It supports PPTP and OpenVPN on macOS Sierra and High Sierra:
https://www.flowvpn.com/download-mac/
Just overwrite the FlowVPN server address with any server and it connects.
For the big picture: I solved the issue by uninstalling homebrew and then re-installing it again. In order to help people maybe gain a bit more insight, let me go into further detail.
Researching the Internet for the error message, my first hunch was that it had something to do with permissions. Check a discussion on Reddit or on the Homebrew GitHub for reference. However, running sudo chown -R $(whoami) /usr/local did not to the trick. In any way, I would advise to be very careful using this command as it will transfer the ownership of /usr/local from the root user to yourself, which, apparently, was not intended by Apple. You can undo this by running
sudo chmod 0755 /usr/local
sudo chown root:wheel /usr/local
However, note that this will only undo it for the folder /usr/local and not recursively for all its files.
So, I was left with uninstalling. If you want to do the same, type
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)"
in order to run the official script or see the Homebrew FAQ for reference.
Be aware that this will also uninstall all your Formulae and that you will be left with installing them manually again later on.
To re-install homebrew, simply run
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
If anyone figures out why I got the error in the first place or how it can be addressed directly, please let me know.
Best Answer
In my situation, setting server-side in ipsec.conf:
solved the problem.
From ipsec.conf documentation:
Moreover, I had to add to the Keychain the root certificate and trust it in order to validate the certificate sent by the server and I read that if you have intermediate certificates you have to trust them too.