Bookends libraries should not be synced by services such as Dropbox, iCloud Drive, Google Drive, etc. Doing so can have unexpected consequences, the most serious being data corruption and, ultimately, an unrecoverable database. This is the first issue covered in our FAQ.
Despite warnings in the documentation, FAQ, and forum, and user notifications when Bookends detects a library in a file-sync service folder, we continue to receive reports of problems caused by this practice. As a result, in an upcoming update (probably 15.0.4), Bookends will simply refuse to open a library stored in a file-sync service folder. Users will be instructed to move the library to another location (the Bookends Folder in Application Support is preferred, but not required), and use Bookends cloud sync instead, which is safe, fast, and secure.
I'm posting this now to restate and clarify the problem well in advance of implementing this solution.
Jon
Sonny Software