Cannot download a family account on Xbox 360

Hey everyone!

I'm having issues with a family account and, I'm afraid, it is rather complicated to describe:

I've set up a Microsoft account for my 12 year old son. That account shows up in my family group on my account's page. Furthermore, I can see the my son's gamertag when I edit the online settings for Xbox on my account's page.

However, when I log into my son's account on xbox.com, I'm immediately asked to create an Xbox profile. When I proceed here, I have to give permission and right after I enter my own password to do so, I get an error page, something like "page not found",
which is very weird for an automated process. I doubt that this is due to the profile already being created because I had this very problem many times before the gamertag was set up.

Still, since I can see the gamertag as described above in the online settings for my family, I thought that I could download my son's profile to the console. But that doesn't work either: I can download my own profile. And my son's gamertag shows up in the
selection on the console as well. But, when I try do download his profile from that selection, it shows "Hello [my son's gamertag]" and the progress bar, only to stop after 5 seconds with error code 80150016 saying that there is a problem with the authorizations
used to log in (in german: den Berechtigungen, die für die Anmeldung verwendet werden).

If I try to download my son's profile by logging into his account on the console rather than my account, the picture is very much the same: it shows "Helly [my son's gamertag]" and stops downloading after 5 seconds, this time with error code 8015190A.

I've done some research on those error codes. I've cleared the console's cache and rebooted several times. Since I cannot log into my son's account on xbox.com (see above), I cannot change his avatar to re-synchronize his account either. I've spent about
6 hours in chat and 2 hours on the phone with the support. Everytime they said they would "escalate" the problem internally and come back with a solution. One time, that soulution was to start from scratch with two new accounts -- which I did, without any
substantial improvement. My last try was to start from scratch on the console, not an the accounts pages, with two new email adresses. The problem remains the same. I tried on three different browsers, one of the being Edge, and in two different home networks.
The result is always the same.

So, here are my questions:

1. Does anyone of you have an idea on how to solve this?

2. Has anyone successfully set up an Xbox profile for a minor kid via xbox.com? If so, how did you do this?

3. Has anyone successfully downloaded a minor's Xbox profile to an Xbox 360? If so, how did you do this?

I'm happy for every input whatsoever. Thanks!

:)