adding a bookmark makes all my bookmarks disappear

I am running Yosemite and today I bookmarked a site by dragging the icon next to the URL to a folder in the Favorites bar. Within a few seconds of doing that, every single favorite disappeared from the bar. I restored the bookmarks from a backup, tried adding a bookmark again, and they all disappeared again. Then I tried turning off iCloud syncing in the System Prefs, restoring the bookmarks, and then turning on iCloud syncing. The bookmarks remained until I tried adding a bookmark again, at which point they disappeared. I then went logged into an iMac and a Mac mini that are both running Yosemite and which both sync Safari bookmarks and they immediately reverted to the Apple defaults in the bookmarks bar. When I went to Safari on my MacBook Pro (which is the machine on which I have been doing all this), the Favorites bar reverted to the Apple defaults there too.

I have since restored all my bookmarks, but I still can't add a new bookmark to the Favorites bar without them all disappearing. Does anyone know what the problem might be? It seems to be iCloud related, but I have no idea how to fix that or where to even begin.

Thank you!