This is according to a tweet by Dropbox who cites that iOS 8 incompatibility issues is what is causing the problem. This means that your files, such as photos or videos, will not be automatically backed up. You can still upload them manually but if you’re used to the automatic backup feature, then you’ll want to be a bit diligent about manually backing up your files.
The good news is that this isn’t a permanent problem as Dropbox claims that they are working on an update that should resolve the incompatibility issue. No word on when the update will be released, but hopefully it shouldn’t take too long. In the meantime iOS 8 is scheduled for a release later today, so if you’d rather maintain the Automatic Backup functionality, then you’ll want to hold off on updating for now.