I read here that Fastmail works well for webDav. Since I'm already a fastmail user this comes as good news.
It took me a long time to figure out the right server, username and password, so I'm posting it here in case it helps someone else.
First go to 'Files' in fastmail and create a folder where you want your Joplin data. I created a folder called Joplin. You will then also need to create an app password (using the Dav / FTP type).
Within Joplin, go to settings > syncronisation, use these settings:
Server: https://webdav.fastmail.com/username./username.your.domain/files/Joplin
Username: username#your.domain (You need to replace the @ sign of your full username with a # sign
Password: the app password (not your main account password)
References: