Michel,
Have the firmware handy on your hard drive of a wired computer.
1. Hold down the recessed RESET button and continue holding ...
2. ... unplug power to the router
3. ... count to 10 seconds ...
4. ... restore power to the router
5. ... count to 10 seconds...
6. Release the button -- allow the router to complete its process to restore the factory default (around a minute, let the lights guide you)
7. Now without holding the RESET button, do one more power cycle (unplug ... 10 seconds ... restore power)
8. Log in to the router's configuration pages, go directly to Tools - Firmware and re-install 1.21 EU, after a successful update, the router will restart -- allow the router to complete its process to program the firmware (around 2 minutes, let the lights and web pages guide you)
9. Now without holding the RESET button, do one final power cycle (unplug ... 10 seconds ... restore power)
10. Your router should be ready for use.
If this helps, it's something a few people have reported before that many believe have something to do with left-over data or bad state happening post-update or post-configuration, or having something in data pre-update. The extra power cycles between those moments eliminate that data or those states.