That message popped up when trying to join the GT Endurance Trophy yesterday. Confirmed by 3 Aston Martin drivers, so I'm assuming it's an issue with (one of the) Aston Martins. In my case I picked the first Aston Martin on the list and haven't done anything with regards to a custom team as far as I'm aware.
you surely have skins that are not official in the game that loads before the original ones it often happens when you download skins
I had the same problem yesterday when joining the GT Endurance Trophy with Aston Martin GTE #55. I have not installed any extra skins. The Aston Martin GTE #95 is twice in my car list. Is this normal?