We recommend against placing a database in Dropbox and running it there. There are entirely too many possibilities for database damage to make this safe.
Instead, the new Sync procedure places a “Store” of database files (not the database itself) into Dropbox if Dropbox is chosen as the sync vehicle. Currently, Sync is specifically tuned to the idiosyncrasies of Dropbox, and may or may not work if another similar service such as Box is chosen as the transfer vehicle.
If you encounter a problem with a Sync attempt, check the message in the Activity log (Window > Log). For example, Sync will (by design) stop if it finds that a database has verification errors. In that case the proper thing to do is to clean up the database before trying Sync again. DEVONthink provides tools to do that, including Tools > Restore Backup or Tools > Rebuild Database.
When the instructions tell you to “clean up” the Sync Store, e.g, in Dropbox, that means simply to delete the existing Store and create a new one.
Please read the instructions carefully. If you encounter problems up to and including a DEVONthink crash during Sync, send a message to Support that includes the logs created by Sync, rather that the normal DEVONthink application crash report. Those logs will be useful to the developer to analyze what happened during the sync attempt.