If you notice my follow up comment was about Cakewalks advertising not you. It wouldn't have been the first time someone got bitten by misunderstanding the recent change in the naming convention of Sonar versions. Glad it didn't bite you.

As to the other I don't think there's anything the forum can do for you except recommend you try sending a PM to one of the Cakewalk staff who frequent the forum and see if they can't help sort out the mess. Sounds like the Serial number you have is not appropriate to the area from where your trying to register. I see you mention being in France till June. If you're there now then you might need to log into the usual area for your Cakewalk account.

I remember someone else having a very similar problem to the OP.

Try the registration page instead of redeeming a product.

https://www.cakewalk.com/Register

Well, hopefully X3e will be a positive change from your X1 experience, and perhaps we'll see you around here and not always at your otheR hangout.  
 

Haha!  I see using the Reaper Word is still blasphemy round here!
 
FWIW I also bought Presonus Studio One Pro 2.
Had a good workout with it over several months and could never deal with the cpu hit and a couple of other negatives. It gets used fairly regularly mostly because of its MIDI abilities.
This is one thing I am hoping is still good in Sonar X3.
I may also try to rewire bits of X3 to bits of Reaper too.  Best of both worlds?
 

And also be warned that after you download the X3 you'll have to go through the whole process again to download the X3e patch. Your looking at around 9 Gigs.  And You'll probably have bad luck using a public hot spot. Most have bandwidth limitations. One missed bite in the download and you might have a corrupted version. I'd find a place where the internet is solid. Go buy a bottle of wine and make friends with someone. Library? 

Cactus Music
...download...Your looking at around 9 Gigs...

Surely the basic (not Producer, not Studio) edition must be a lot smaller.

If he had X1 then there's no need to download Dimension Pro as it hasn't changed and that's by far the largest part of the download. SO the remainder should be only about 2-3 gig plus the x3e update.

If you use Chrome when you do your downloads, it has a built-in download manager, which will help ensure your downloads complete properly.
 
Post any issues/questions here and folks will do their best to help you work through things.
 
Bob Bone
 

mudgel
...there's no need to download Dimension Pro...

This won't be available anyway. It's not included in the basic edition.