In case someone ever happens the same ... I have found the solution to my problem.
I think it is a bug but what I don't know is if it is from Assetto Corsa, from Content Manager, from Custom Shaders Patch ...
For RoC tracks (Race of Champions - 1 vs 1) we cannot put in the "ui_track.json" some "pitboxes" = "2" even if that is the correct number of cars participating in that type of racing and the necessary pits ... we must put "pitboxes" = "4" even if we have only two pits in our 3D file (AC_PIT_0 and AC_PIT_1).
In addition, the race must be configured as follows in the Content Manager so that a crash / error does not appear:
-The starting position should not be random, we have to choose 1st or last because if we choose random it is possible that the two cars appear occupying the same place.
-The number of opponents must be 3, although there will only be 1 vs 1 on the track (because in our 3D file we only have 2 AC_PIT and 2 AC_START).
-The number of laps must be 2, 4, 6, 8 ... always a pair number (so that both cars travel the same distance, because this is a special type of track where the track intersects).
This is the solution I found, after doing a lot of testing with my track and with other RoC tracks.
It's kind of weird and doesn't make much sense and I think it's a bug, but it's the only solution I found. I hope it helps if you ever do a 1 vs 1 RoC track.