I just wanted to document what I had to do in order to get Joplin syncing up with NextCloud. I am using the Linux client, iOS client, and Windows client.
My primary reason for setting up NextCloud was to use it for syncing with Joplin, however I also am using it for backing up photos from iOS and as a replacement for DropBox for syncing files between household phones, servers, laptops, desktops, and various media devices (most Raspberry Pis). I am also replacing things like NFS/Samba with it, so it's become more valuable now that I have it setup. Anyway, onto the details for Joplin.
The most recent advice I could find on the forum or otherwise didn't work with NextCloud 21, but did work on NextCloud 20.
With NC 21, I needed to create the /Joplin directory in NextCloud along with a /Joplin/locks and /Joplin/temp directory. Before I manually created these directories, I was getting various errors when trying to sync, the error varied depending on which client I tried.
The URL I supplied was:
http://hostname:port/remote.php/dav/files/username/Joplin
After those changes, I was able to sync notes I created on each client and they showed up correctly on each client.
I hope that helps someone else get up and running faster.
-Chad