Unfortunately, that does not seem to be supported by Nextcloud - I get back
<?xml version="1.0" encoding="utf-8"?><d:error xmlns:d=“DAV:” xmlns:s=“http://sabredav.org/ns”>
<s:exception>Sabre\DAV\Exception\Conflict</s:exception>
<s:message>PUT is not allowed on non-files.</s:message>
</d:error>
I’ve also tried curl -X PUT -u login:password -F "Content-Type : text/calendar; charset=utf-8" -F "file=@plop.ics" http://example.com/SUBDIRECTORY/remote.php/dav/calendars/USER/reminders/plop.ics
but that resulted in
<d:error xmlns:d=“DAV:” xmlns:s=“http://sabredav.org/ns”>
<s:exception>Sabre\DAV\Exception\UnsupportedMediaType</s:exception>
<s:message>This resource only supports valid iCalendar 2.0 data. Parse error: This parser only supports VCARD and VCALENDAR files</s:message>
</d:error>
To make certain that this was not due to me messing up the file while copying from the forum, I also tried the same with an appointment that I had previously downloaded from the same Nextcloud calendar, but same result.