Thanks a lot 4Derson! After trying the old solution and being unable to save the modified .plist, I tried to find the code on your webpage and it worked! :-)
It was not clear at first to me so I'll explain my way:
No need to edit any file, you can simply find the actual password from your backup! I used iBackUpBot (demo version) and in the backup files matching my device, I opened the following folder: HomeDomain > Library > Preferences. Then I opened the file com.apple.restrictionspassword.plist.
Then you simply identify the two strings that look similar to the example shown on that page, copy/paste them in the appropriate text boxes and then scan for the code from 0000 to 9999. As the scan is slow, you might want to try a few possible codes you might think of and see if the keys match. And you can stop or resume the scan from/to the point you had reached before. Finally, you will find the code stored in the device and simply enter it the normal way to unlock restrictions!
Hope that's clear for those who, like me, didn't understand the principle at first.
Oh, and to those who are wondering if it's their memory or a bug... In my case, even if I was pretty sure I had tried all codes I had possibly been using, as soon as the scan revealed the code in use, well, I then could recognize and remember the code I had chosen myself and properly fogotten—so not a bug for me!
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