Comments - Etnamita

A similar thing happened to me too I can suggest you check the UTEngine.ini file, the one you find in the folder:
C: \ Users \ username \ Documents \ My Games \ Unreal Tournament 3 \ UTGame \ Config ... and to the URL entry add the string
LocalMap = UTFrontEnd_MapMixer.ut3
;LocalMap = UTFrontEnd.ut3

Maybe it's a bit late to ask for help, however Mapmixer is really useful, the only thing I couldn't understand is why when I try to do team deathmatch with custom skins I always get the default skins.
More precisely:
I try to create teams with various skins, and I noticed that if I have a custom skin, the other teammates and opponents all have the same default skin.
If instead I select an original skin of the game (like Iron guard, reaper etc.) and for the others a custom skin, then it loads them all right, but I stay with a default skin.
I've even tried as a spectator and the skins load them correctly, but if I enter the game with a custom skin, everyone else has the same default skin.
This does not happen in regular Deathmatch, only Team Deatmatch.
In any case, Mapmixer is a really nice mod. even with this small flaw.

Rectification:
I found a solution, the problem is not related to Mapmixer but to the game itself.
In Epic's forum search for "UT3 Community Edition Update (2020)", it is a set of solutions for a clean install of UT3 and also includes a Mutator to avoid the tedious skin problem, it's called "No More Demo Guy".