Thanks for the detailed reply, @Bernie_O, and no worries about the delay – we’re all volunteers here and help is always appreciated.
I had previously tried to follow the post’s steps but ran into the error I mentioned above. Since it now appears the error was my own, I will try again.
Beyond this, I am indeed using Little Snitch. Good to know this may be a cause for concern here; I’ll keep an eye on @factor4’s reply.
Best,
EDIT: I already wrote on the Blogpost’s page 
I saw your edit just now. Unfortunately something went wrong and your comment seems to not get stored on the Blog-Server.
Are there any news? Did you get the reminder sync to work?
Hey @Bernie_O, thanks for following up on this here and sorry the message did not work out.
So I do indeed get stuck. The first commands and the creation of the folders/files goes well, and I personalised the openssl-server.cnf file as necessary (I think).
However, the next command (openssl req -x509 -config openssl-ca.cnf -sha256 -nodes -out cacert.pem -outform PEM -days 7300) gets the terminal to hang indefinitely. It’s not actually failing like before (I am indeed in the right folder — /Download/SSL2, for me) and there’s no error message, but it just hangs and that’s it.
Any ideas? Is it an issue that I am using zsh?
I can confirm that the command just hangs under MacOS and I have no idea why. MacOS uses libressl - a fork of openssl (you can check that with openssl version). According to the developers the api should be identical to the openssl api, so the commands should actually work. As I am always using a linux box to create the certificates, I am very sorry that can’t help here.
No, I don’t think zsh is the problem here, as it does nothing else than invoking openssl (aka libressl).
P.S.: meanwhile I found your blog-comment in the spam and was able to restore it!
Ok, then it’s not just me, that’s always a relief.
Actually, following your earlier comment, I checked my version of openssl and, indeed, it’s libressl 2.8.3 (if my memory is correct). I therefore installed homebrew and openssl, to try openssl’s most recent version, but macOS keeps on using libressl. Would you know how to tell macOS to use the installed openssl instead? Maybe that’d be a fix.
homebrew installs the openssl binary in /usr/local/opt/openssl/bin/. You can check, if it is working with
/usr/local/opt/openssl/bin/openssl version
If that outputs something like OpenSSL 1.1.1i 8 Dec 2020 you are ready to go. Just use the full path to openssl for each command. The first command would then look like:
/usr/local/opt/openssl/bin/openssl req -x509 -config openssl-ca.cnf -sha256 -nodes -out cacert.pem -outform PEM -days 7300
Let me know if that works!
It does, thanks! I’ll keep going and ping back when all is done or if things get stuck again.
Well, that did not take long. Of course, now all the openssl commands work successfully.
Next step is to add servercert.pem and serverkey.pem to my server. Any idea on how to do this for Webo Nextcloud Hosting? A cursory look at the nextcloud support pages did not yield much information 
…
No, sorry. I don’t use Webo. You have to ask your hoster.
So there is good news and bad news.
The bad news is that I have not heard from my hoster, and that, therefore, this whole process of creating the CA has, for now, been in vain.
The good news is that, somehow, the sync now works. I do not have a real explanation for that. I just updated macOS (to version 11.2 20D62), but since I had not checked the sync in a little while, I cannot say for sure that this is what made the difference.
At any rate, thank you all for the help and I’ll keep an eye on the sync in case it breaks again.