It’s a problem with the WebDAV instance you’re syncing with (I’m not intimately familiar with Sabre DAV), for some reason it doesn’t seem to send the required authentication header. So you need to check that (permissions, setup, etc.), reinstalling Joplin will not solve this as the issue lies elsewhere.
Edit: This topic is about the same issue with a solution (that is, basically, you need to have a certificate for https): Network request failed ERROR trying to sync Joplin iOS against NextCloud 18 - #5 by mowic