I’ve set up WebDAV sync centered on a synology NAS, which I’ve confirmed is visible on the WAN outside my LAN. I can connect via WebDAV to both iPhone and iPad using other apps, so I know the problem isn’t with my WebDAV setup.
when I try to sync with the Synology data store from either my iPhone or my iPad, I get the above mentioned error. Found nothing here searching on the error term.
On the Mac, in DEVONthink’s Preferences > Sync, right-click the Sync location and choose Clean Location. This will remove the old Sync data so you can Sync afresh.
I did as you suggested. Now I’m getting “invalid encryption key” errors. I’ve ensured that the keys are in fact identical across the various iOS and MacOS devices.
Do the Clean location.
If you run into the “path exists” issue, create a new Sync location with a new name and Sync to it. After that you can abandon the old one.