When you get the error, where in the paper path are you removing the paper from? This error actually indicates the paper did not pass through the Exit Sensor within the specified period time. The Exit Sensor is in the fuser
You said when you removed the Duplexer, all is well...... so we must assume the exit sensor has no problem.
The Duplex Unit has it's own motor drive and therefore, controls the speed at which paper moves through it. If you see the Duplex unit listed as an option on the settings page..... and you have verified it is installed correctly..... I think we can safely say that duplex unit is defective.
(check the Duplex Unit plug/jack connection to ensure that none of the contacts were damaged during installation)
We can test these motors using the built-in diagnostics. Do you know how to enter diagnostics mode on the printer?