@swanny okay, reading the thread through carefully it does seem that it’s a driver problem rather than motor; it’s always driver 3 that reports the error. Unfortunately some of your answers confused this, but I think it’s just language!

From your config.g, it looks like you have spare drivers on your Duex. With power off, disconnect extruder drive from driver 3 (E0) and connect to driver 8 (E5) on Duex, then change M584 in config.g to:
M584 X0 Y1 Z2 C7 E8:4:5:6
Reset for changes to take effect. That should get you going again.

Then please take a good, high resolution picture of both sides of the Duet board around the area of the driver, and post them here.

Regarding warranty, the European 2 year warranty is not a full warranty for 2 years. See https://europa.eu/youreurope/citizens/consumers/shopping/guarantees-returns/faq/index_en.htm
Most relevant answer is this one, which explains you are covered for manufacturing faults for 6 months, which we have extended to a year:

I bought a phone just over a year ago, and it has stopped working. The trader refuses to fix it for free. Do I have a two-year legal guarantee?

The legal guarantee is valid for a period of two years throughout the EU. But there are certain conditions.

The legal guarantee covers any defects presumed to have existed at the time of delivery and which become apparent within a period of two years. However, the crucial period is the 6 months after you bought your product:

Any fault that appears within 6 months will be presumed to have existed at the time of delivery. The seller must then repair or replace your phone free of charge, or reimburse you if repair or replacement is impossible.
After 6 months, you can still hold the seller responsible for any defects up to the end of the two-year guarantee period. However, the seller can ask you to prove that the defect existed when your goods were delivered. This is often difficult, and you will may have to involve a technical expert.

I assume that you have been using the Duet without problems for over a year?