Oh sweet Mary and Joseph…It took me a long time to make my sync scheme works, and now it’s all gone…
So, I sync form my Macs to the Synology that sits beside my computer…
It was setup as in the first figure…now I’ve tried everything in the new setup window and get all sorts of errors…error 400, method not allowed…
The URL now is preselected…tried the old url too…
Any ideas…? Please… 
I am actually having a similar problem. Trying to Sync to a Synology NAS via WebDAV.
I get this error message: Error while setting up location “DevonThink.dtCloud”
method MKCOL not allowed (405)
On the NAS the error message that shows up is: 2016-07-22 15:28:02 Warning JamesNAS SYSTEM Connection User [jamesgalvin] from [192.168.0.101] failed to log in via [WebDAV] due to authorization failure.
I am using the correct username and password and I can connect via WebDAV through the finder.
Here are my sync settings:
URL: jamesnas.
Username: jamesgalvin
Password: *
Sync Store Name: DevonThink
Encryption: *
First 3 boxes are checked and the last one (verify) is not.
Any ideas?
I don’t know if it was my particular setup, but the key for me was putting /home on the end of my Synology address. I have a custom URL, so the address that worked for me was this:
Note that “5006” isn’t what I used, but thats only because I use WebDAV over a different port.
I’ve uploaded a few non-senstivie databases from my MacBook with no errors. I’ve tried syncing on the other end, from my iMac, only once. I got no errors. I’m going to try the rest when I get home tonight.
It actually cut off the rest of my URL (possible because I put the word hidden in brackets for the domain name). I used the full URL and path though. I use x this time to demonstrate:
jamesnas.x.net:5006/home/DevonThink
Great! That worked. Not for the local name, but did the trick with the full web path…
Too many variables, wish it figured out by itself, but anyway…
Thanks man! 
Well, I’m glad that it worked for you. I’m still getting the error message. Lol.
I was able to get it to work. I apparently did configure it correctly, but it must have had an old value in memory or something because I tried quitting the application, reopening it, and trying again, and it worked.