Might be a silly question but do you definitely have the 'Joplin' folder on it and if so does your Nextcloud URL in Joplin also have it i.e. https://nextcloud/remote.php/dav/files/username/Joplin
@Daeraxa No that's certainly not a silly question and thanks for your reply... I checked, checked, and checked again and again.... and everything was in place, the address, username & password. What I did notice was that I couldn't get a connection to CalDAV/CardDAV either using DavX5 or other cal/CardDAV app.
The good thing was that it was a new install and not fully set up... (I have a habit of checking things over to make sure things still work each time I add a new app or function to Nextcloud...)
I did a reinstall of NC and now everything is working as advertised... Whew!..
So all good thanks, and again thanks for reading and answering...