AdBlock removed Dropbox sync in March 2016. If you're using that feature after March 2016, you need to update your copy of AdBlock. :)

In the meantime, you may see this message:

You have exceeded the file size limit of Dropbox. Please remove some items from your custom filter list, and try again.

AdBlock used Dropbox's datastore API, which limited us to 100 KB of storage. That's not enough, which is why you're getting this error. If it's really annoying, you might want to disable Dropbox syncing. Or, you know, update AdBlock.