How could this happen?! An investigation about a 40€ loss due to unlikely circumstances

I think you'd only have a shot at a refund if the code was claimed on a date before you purchased it. Since it was used a while after you paid for it, the responsibility would be out of their hands. For example, if you purchased a physical copy of a game, want to sell it on ebay and realize the day you want to ship it that it has been opened and the game disk or cartridge was missing, you couldn't hold amazon at fault since it wasn't shipped that way. (Sorry if that analogy sucks haha.)

I do think it's a peculiar thing to happen but since the code was used just before you tried to sell it rather than shortly after you obtained it, I don't think amazon had anything to do with it. The internet trouble around the time sounds suspicious but isn't much to go on about. And I doubt you need anyone telling you to not connect to anything needing a password when on an unsecured network, so lesson learned I guess?

Still if you happen to do find out more I want an update.

Since its a sale, like publishers/retailers they often mixed up the codes. Your code may have been given to someone else again and since you did not redeem it immediately it was redeemed. Personally, I'll to dig with Sony for more details.

I do not believe anyone will hack you in your building unless they know you're a big youtuber or online personality that chances you have codes hanging around. Unless you're in a work in real life that attracts anyone's interest and you have shown too much information to others.

Do you share your computer with someone else or if you did not set a personal password, who could have excess your computer? Perhaps its someone you know or unknowingly left the computer unattended and was breached in that manner.

Edit: Is your email 2FA protected?
Cheers~

Anything and everything on your computer can be accessed if you connect to a public network. That includes Chrome cookies and temporary internet files. Any half-assed attempt of data-mining through a network connection could steal some, if not, all of your data.

It's highly unlikely that the code given by Sony is at fault. Amazon has built-in systems that blocks the same codes (even with space differences) on any CSV file.

Another possibility you didn't mention is that it wasn't actually sold by Amazon but a third (marketplace) party? If so then anyone from this company could have stolen it. Or wherever they got the code from was an unsafe source. Or someone else could have gotten access to their PC(s). Or they just send the same code out twice to two different customers (by mistake (or not)).

You were doing business on an unsecured public WiFi connection. That's almost certainly the culprit.

Check your browser history to see if any of those site you were using on the free network were downgraded to HTTP.

Technical details for the geeks: https://avicoder.me/2016/02/22/SSLstrip-for-newbies/

Without exploring all the possibilities (and they are many), the most likely one might be that you created an account on a third party site using the same password used by your e-mail therefore granting to whoever manages or has access to that site's database free access to your e-mail. Any place that uses the same password, no matter when the account was created can possibly be used to gain access to your email and therefore to any codes and further data stored in there. Try to use unique and strong passwords whenever possible and change them periodically without using easy to guess patterns.

This is certainly really interesting and reading through the comments, I don't think I have anything to add but good luck in obtaining a replacement code :)

To not repeat the others... brute-forcing a key like that seems really unlikely. Chances are drastically small.

Well.I 've got a reasonable yet useless analysis for you.Of course,it's just for fun.
Cause what's gone is gone,and this kind of mishap is very unlikely to happen to you ever since.If not,probably my point works.

That is,you have another personality whose existence is unnoticed by your usual self so far.And that personality happened to want to subscribe for the PSN service under his or her own name.

Case closed~ And I have suddenly got an urge to check if people can actually complete a series of tasks that require certain concentration and consciousness while sleepwalking.

AN UPDATE!
After a dozen phone calls and emails with Sony and Amazon, I found myself in a dead end. Both rejected their fault on each other. I couldn't get more information from Sony about the thieve, telling me it was confidential. At first, one Amazon guy tried to obtain a refund, but it was denied.
A month later, I contacted Amazon again to try my luck. Glad I did. I finally managed to get a refund on my purchase. Case closed! I will never know what happened, but at least I got my money back.
Thanks to everyone who replied, very much appreciated! It was fun to know your opinions. Enjoy your holidays :)