This is very similar to:

https://bugs.mageia.org/show_bug.cgi?id=4156

but I believe it's not the same. The easiest way to replicate
this is to set up two Mageia boxs with a local repo(s). They
would have M2 or M3 or both repos on both boxes. I put the
repos in user "mageia". My repo source is mirrors.kernel.org.
So my local repo will be at:

/home/mageia/distrib/

LAN IP is: 192.168.1.2
Lets make the password: linux

I have also installed Apache on two ( M2 & M3 ) drives such that:

/http://192.168.1.2/~mageia/

exists. In the mageia public_html directory I have created
the directory /public_html/repo and in that repo directory
I have created a soft link back to /home/mageia/distrib/
So what that creates is the same repo at both:

ftp://mageia:linux@192.168.1.2//distrib
http://192.168.1.2/~mageia/repo/distrib

But getting to the ftp repo requires an ID & PW Authentication.

My test server features a removable replaceable HD system so
I can A <-> B HD's with either M2 or M3. I have created two
such drives with working repos for both M2 and M3. Both
drives have user mageia ID:mageia PW:linux. The server LAN
IP is: 192.168.1.2. The M2 drive is actually the one I have
been using since M2 was released. The M3 HD I created right
after M3 was released. I copied the M2 & M3 repos from the
M2 HD to the M3 HD and rsync'd that up with the mirror at:

mirrors.kernel.org

Using a client workstation I turned off its HD and am using
only the Live-CD's:

M2 Live-CD 32-bit KDE
M3 Live-CD 32-bit KDE

Boot the server with M2 on it. Boot the client with the M2
Live-CD. You can set up the clients repo with either of the
following commands:

First in an su terminal execute a: urpmi.removemedia -a

Then execute either of the following commands:

urpmi.addmedia  core  http://192.168.1.2/~mageia/repo/distrib/2/i586/media/core/release      with media_info/hdlist.cz
urpmi.addmedia  core   ftp://mageia:linux@192.168.1.2//distrib/2/i586/media/core/release     with media_info/hdlist.cz

The client will make contact with the server and properly set up the repo.
You can set up the entire repo set using either the http or ftp process.
This process works for both the M2 & M3 Live-CD's.

Remove the M2 drive from the server and replace it with the M3 drive.
Reboot the Client using either the M2 or M3 Live-CD.

In an su terminal execute a: urpmi.removemedia -a

Then execute the following command:

urpmi.addmedia  core  http://192.168.1.2/~mageia/repo/distrib/2/i586/media/core/release with media_info/hdlist.cz

The client will make contact with the server and properly set up the repo.
You can set up the entire repo set using the http process.
This process works for both the M2 & M3 Live-CD's.

In an su terminal execute a: urpmi.removemedia -a

Then execute the following command:

urpmi.addmedia  core   ftp://mageia:linux@192.168.1.2//distrib/2/i586/media/core/release     with media_info/hdlist.cz

This will result in the error:

...retrieving failed: curl failed: exited with 9
no metadata found for medium "core"

Effectively indicating that "The server denied login or denied access to the particular resource or directory you wanted
to reach"

If you replace curl with wget then the command will be:

urpmi.addmedia  --wget  core   ftp://mageia:linux@192.168.1.2//distrib/2/i586/media/core/release     with media_info/hdlist.cz

This will result in the error:
...retrieving failed: wget failed: exited with 8
no metadata found for medium "core_"

Effectively the same access error.

The same error will be with both the M2 & M3 Live-CD's so long as the server is running M3.
But if the server is running M2 everything is fine.

I believe that bug 4156 was in the boot.iso code. This time it looks to be in the OS's code itself.

Sorry, I do not have another M2/3 repo source that requires authenticaton available to me to test.
I suppose you could set this up using Vbox but that would be huge as the repos are over 50GB in size.

Using the boot.iso for either M2 or M3 will have the same results. An M2 server will
be fine, and M3 server won't work.

Reproducible: 

Steps to Reproduce: