Unknown error is an unknown error.
Basically what it commonly means is that updater.sh (script handling updater’s execution) detected that pkgupdate exited with error but no error was reported by it. This can be because pkgupdate failed big time or just because there is something that we forgot to catch in updater it self.
What you have to do is to run updater in terminal to see what is really happening. updater.sh (or updater.sh -e DBG for debug output).