It seems that I had too many failures. The range of IPs I belong to was somehow greylisted on Huawei's servers. I guess that's why, at some point, I couldn't display the unlock agreement page, even clearing the history of the browser, using another browser or using another system…
I stayed away from Huawei for a couple of days. Today, I took into account all the advices : chinese version of the site, Chrome on Android (history cleared) with desktop version required and translation set on.
- on my desktop, I created a new account on club.hihonor.com/in/index which sent me an email containing a good validation URL and confirmed the account creation
- switching to my android smartphone, I pointed Chrome to https://www.emui.com/cn/plugin.php?id=unlock&mod=detail to get the Unlocking Agreement
- went through ta log in page (where I could log in with the club.hihonor email-address+password I created just before)
- reached the unlock page (step 7), filled the form, validated and got my unlocking code.
Thanks for the method and the additional advices