I don´t know if this is an item associated with my Synology WebDAV-server setup or a Devonthink item.
In my system if I want to set up eg a WebDAV sync location I go to Preferences -> Sync. In the list I will find my server, select it and in the resulting window the URL-field contains the preamble for the URL. However, an extra “.” is included in the suggested string, eg my URL is listed as “https://diskstation2.local.:5006/”.
At least for me this address (when the rest of the URL is filled in) will result in error “Bad request (400)”. If the “…local.:5” is changed to “…local:5” everything works as expected.
This behavior is the same in the iPad/iPhone setup.