This is the most important thing to remember when using this method of syncing Lightroom catalogs between multiple computers. Forgetting this will result in problems such as duplicate conflicted copies of the same catalog or corrupted catalogs that cannot be repaired.
Before you open a synced catalog, you must, must, must make sure that Dropbox has finished syncing on both computers. If you open the catalog before the file syncs, Dropbox will corrupt the catalog file.
When this happens you will get an error from Lightroom that tells you that the file needs to be repaired. You can try to repair the file but it will not work. Your catalog file will be done for.
Don’t worry though! There is a super easy fix if this happens (I should say, “when it happens” because it will happen. It’s super easy to forget and then remember when it’s too late). Backups are very important and Dropbox will thankfully keep a backup of each of your files in case something like this happens.
Simply go on to Dropbox’s website and go to that file, get a backup from the version history and restore that file. The backup will now push to your folder. Once it does, you can now go back to using Lightroom normally.
Lightroom catalog and preview files are currently being synced, as noted by the blue circle.