What specifically happened between the time it worked, and the time it didn't? Sometimes that can point to where the problem is.
Some of the C600 manuals (like Dapu) show error 9 as overvoltage, > 45v. What voltage is the battery?
If it's really a comm error problem, then that is something between display and controller, where one can't talk to the other, usually that the display is not receiving any info from the controller.
Sometimes that's because the display hasn't turned the controller on, meaning the controller isn't receiving battery voltage on the Lock / KSI line, so it doesn't have power to the LVPS that makes 5v/etc to run things from. If you can set your voltmeter to 20vdc, put the black lead to battery negative or any other system ground wire, then disconnect your throttle or PAS sensor, turn the system on, and on the controller side of that connector put your red meter lead on each of the three pins in turn. At least one should read 5v. If none do, the controller is probably not being turned on.
If that's the case, there is some connection or wiring problem between display and controller on the lock line, *or* the transistor inside the display that does the switching for this has failed, like in this thread:
I'm helping a neighbor diagnose his bike, which displays an Error 30 a few moments after you power on the bike. I found dozens of posts about this particular error across several websites, videos on YouTube, and a troubleshooting guide on the bike manufacturer's website. So, I really didn't...
endless-sphere.com
Sometimes a comm failure is literally a comm failure--something has gone wrong with the TX or RX between the two, either a transciever failure inside either controller or display (or both), or a connection / wire problem. Doesn't even have to be a broken wire--it can just be poor contact from a spread pin inside a connector, or corrosion,etc