you can test it with wget <ip-address>/.well-known/caldav and see some redirections. “Outside” could even be a ubuntu terminal/shell. But I don’t think the test is wrong.
That one is a tricky one… As it sure is not certain, that you need the function those rewrite-rules give to you… You don’t have problems on “the system” but with probably attached computers/mob-phones… that would be: problems setting up calendar and contact apps. (So: problems elsewhere and the fix on “the system”)
But something else - there are more requirements for this .htaccess files to work. If you like documentation - well Apache HTTP Server is a tough one 
- look here: Apache HTTP Server Tutorial: .htaccess files - Apache HTTP Server Version 2.4
Navigate to the botton to the troubleshooting section and read:
When you put configuration directives in a
.htaccessfile, and you don’t get the desired effect, there are a number of things that may be going wrong.
Most commonly, the problem is thatAllowOverrideis not set such that your configuration directives are being honored. Make sure that you don’t have aAllowOverride Nonein effect for the file scope in question. A good test for this is to put garbage in your.htaccessfile and reload the page. If a server error is not generated, then you almost certainly haveAllowOverride Nonein effect.
I like that test! 
tried with coffee leftover, banana peel etc. - didn’t work 
That sentence: " Make sure that you don’t have a AllowOverride None in effect" … That is not trivial: you have to look at least in /etc/apache2/apache2.conf and all the files in or under /etc/apache2/sites-enabled/
Another edit: The documentation says
there are a number of things that may be going wrong
Well that is so reassuring. So there are after a certain time some questions:
- is the sun shining? And should I go out for a walk?
- do I need this subfolder? That is: will I run anything else than nextcloud on this webserver? Perhaps I could go then with the answer of @magber68 (if I understood that correctly) → the problem then would be different then: how to change the “document root” in the apache configs - or how to setup a good vhost-file for apache. But I think this is much better documented in nextcloud docs then the few words for troubleshooting this redirect issue.
- do I want to learn more about this patchy webserver? Solve false problems and learn something rather than avoid problems and learn little
Best, Bernd