You can’t open a sync store via the File > Open Database command. It’s not a database to be opened. It’s a sync store containing raw, chunked, and optionally encrypted sync data about the database.
You need to import the database from the sync location to the other devices…
Sync is covered in the Help for each application:
- For DEVONthink, sync is covered in the Help > Documentation > In & Out > Sync, Preferences > Sync, and Troubleshooting > Sync Issues.
- For DEVONthink To Go, sync is covered in the ? > Help > Sync your databases section.
There are manuals available here: Manuals
There is also a forum post on syncing: Sync Types Explained