I bought an used Variax 700 bass recently and found out that the firmware was not updated. Tried 2 times through my helix using the cat5 cable, same error -> code 8000900C.
Tried a PC restart as well, new update trial, same error. Then I remembered I still have my USB interface from when I had a Variax 500 guitar. Plugged it in and to my surprise it worked like a clock even though I run on win 10 64 bits. Happy days! (I don't know if the C at the end indicates another type of error though.
You have to remember to power your Variax and if you are using batteries (I used fresh ones), just use a regular jack cable to 'ignite' the variax.