What causes error code 091-720 on a xerox colorqube 8900x printer?

https://www.support.xerox.com/en-us/article/en/x_cq8700_en-O5747

Color Qubes are the most difficult printer to support,
A: Without a proper support manual.
B: Without looking at the problem first hand.
C: Without a supply of spares.
Hopefully you have a maintenance contract with Xerox.
I don't remember all the error codes of the top of my head, but I do know they can be a real problem for any of the engineers.
Good Luck.
Dave.

Appears to be a marking unit issue.

Interesting. Recommend reading and trying the software from Microsoft. http://www.saetechnologies.com/colorqube-error-code-093-547/

Solutions for : Fault Code 393.511.00, 093-511, 393.511, 93-511: Cyan ink stick is jammed. Call for assistance. Message Related Fault Code: 393.511.00, 093-511, 393.511, 93-511 Solutions ranked by relevance:

1. Make Sure the Correct Ink Stick is Being Inserted
2. Check if CQ8700/8900 Ink is Being Used With the Machine
3. Power the Printer Off and On, Wait for the Printer to Get to a Ready State, and Then Load a Cyan Ink Stick Into the Printer
4. Perform a Software Reset (For the Copy Controller, Network Controller, or Both)

You have no light output. The system places a charge, the image will remove charge (without image processing, the system leaves full charge ) (removes charge where no image is if negative processing)
EX: a copier has platen closed makes a copy because the cover has a white surface the light is reflected and removes charge from Drum. If you make a copy with platen open out put will appear black. Charge was not erased, toner is attracted to charged area.
Most systems use a small wire to place charge, depending on what development (pos or neg) with broken wire will also result in either a black or blank output. I would look for a broken charge wire.

error code 377.123.02 for Xerox 6700

I looked at the Xerox 8900 support page: http://www.support.xerox.com/support/colorqube-8900/support/enus.html

The answer I found for Error Code 093-516 was problems with Yellow Ink Stick. Our printer had been switched of for quite some time which probably caused the ink stick/sticks to get stuck (or something). I opened the ink bay and pushed slightly on all the sticks to make sure they were "all the way in", added some more ink behind them and it started to work again.

I had to do a few cleaning jobs for smudges and now it's fully operational.

power loss detected.
Try to resend all your faxes or delete them.
Try a different power outlet.
Engineer callout

The error is a printhead tilt fault. It's unable to tilt on the axis, caused by a fault camshaft and process motor. The part you need to fix it is 8570-Z1.

http://www.iteminconline.com/8570-z1_8570-z1-xerox-colorqube-8570-8580-8700-process-drive-camshaft-kit-includes-process-drive-motor-drum-maintenance-camshaft-transfix-camshaft.htm