This error usually appears on iOS devices when trying to link to a device or connect to a shared folder.
It means that the current identity on this device is either missing or corrupted. It may happen in cases when, for example, Sync or device experienced crashes, Sync was not quit correctly, after system update, or restoring from backup, when not all settings might have been saved/restored correctly.
To fix that go to Sync menu -> My devices, find "This device" -> Unlink.
Create the new identity, and after that proceed to linking devices or connecting to a folder.
In case there's no "This device" and My devices list looks blank reinstall the app.