Qnap didn’t admit the problem so no fix was offered. My advice would be to wait and avoid my sleepless nights! When I investigated further I found new Qnap updates appear to be edging towards handling the very latest os versions (not appropriate for me as I tend to hang back to avoid bugs I am still on High Sierra). I was simply pressing the upgrade button from habit assuming that it was correcting bugs and not creating them. I will look into rolling back tomorrow.
WOOOOT!!!
DEVONtech FTW!
/highfive
/dance
(Sorry. Ghosts of my World of Warcraft days overcame me!) 
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Interesting…for the TS-451+ they released the GM of 4.4.1 on 9-18, and updated it on 10-05. No mention of a WebDAV fix, but they note that WebDAV is now disabled by default in both release notes. Think I’ll stick with 4.3.6 for a while yet.
Yep, that’s the model I have too, last two upgrades broke WebDAV hence my warning.
Jim, I looked up your response as I’m simply an old and bald historian… you youngsters will have your fun! Great product though and top support thank you.
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Haha! No worries. And I’m likely not as young as you imagine 
And thanks for the generous words of support and encouragement! It’s very appreciated.
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I have heard that you need to simply ignore the error messages — they are not useful.
Try repeatedly checking and unchecking the sync on the offending databases. The connection fails past a certain point dependent on how many docs are in the databases. Eventually the checking/unchecking will force the sync to succeed. After that if you make sure to sync after every doc added, you won’t see a reoccurrence.
@Metalman - how were you able to check if QNAP upgrade caused this failure? Any particular tests performed if this was the root?
For the last month i can’t seem to connect to my webDAV…
Just recently upgraded to 4.4.3 as Hybrid Sync 3 was failing and QNAP support would only look at my ticket if had the latest firmware…
I can’t be sure if it was the firmware or not.
Are you still on your QNAP firmware version - if that’s the case?
downgraded to 4.3.6 per post.
Thought it had worked as had no error messages. Tried using the external IP, DNS and internal IP address within the URL. no such luck.
Initially had the:
‘could not connect to server’. NSURL.ErrorDomain -1004
Once down graded then had the: ‘Authentication failed’ message.
After 24hrs+ went back to: ‘could not connect to server’. NSURL.ErrorDomain -1004
On the local database i have 2x db. 2 of them shows last synchronised timestamp with current timestamp. ???
Tried to create a new DB off the existing db - it has Authentication Error whilst trying to synchronise.