I don't know how to say this but either you are blind or stupid.
That server tells everything there:
"Illegal GRF modification detected"
"Please remove any custom modifications and restart the game"
You're making edits on your grf when you're not allowed, get the original grf of that server and replace it for your actual grf to continue playing.
You're lucky that they just make that and doesn't ban you right away seeing how that works.
If you're asking for a way to evade that you're in the wrong place, people here aren't suppose to help people to bypass security measures of servers.
If you want to see 3rd classes at that server then the best bet is to ask at their forums/gm for a quest to just get the sprites as an option for who wants them.
