The portal back into the tunnel works, but the action location in the tunnel out to the arena seems to be gone. I don't think it is related to the textures: Guild house doors inside are normal-mapped too but still work.
I can leave the arena tunnel normally by walking outside. If it doesn't work, one may have an outdated "zoneinfo.xml". A repair might be recommendable, or a re-installation from the 0.5.7 installer package.