Yesterday I was struggling to set up the WebDAV configuration in Joplin with a TrueNAS server I just installed in my home, it ended up being a silly option that was easy to fix. However, during my struggle I stumbled upon this post and I felt bad for the OP, so I wanted to make a very brief tutorial on how to set up your TrueNAS-Joplin WebDAV sync.
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Step 1: Create a new dataset for the WebDAV share (or use an existing one, but be careful)
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Step 2: Create the WebDAV share, select your dataset and click Submit.
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Step 3: Configure the WebDAV service. If you haven't already enabled the WebDAV service, after you create the share you will be asked if you want to enable it; you can do it now or start it manually later.
Here is the part where I failed. By default the HTTP auth is set to "digest", change it to "basic". Set a password and click save.
- Step 4: In Joplin, make a backup of all your notes, open your sync settings and enter the appropriate url pointing to your dataset. The user should be "webdav" with the password you configured:
That should be it! 
@Unis_Torvalds sorry for the late response. 