Also, the STAR can't send call directly to a hunt group, so it first passes through another CRA which immediately times out to the HG. Inspect this CRA, make sure there's nothing strange in the digit translations there. So route would be STAR > CRA > Hunt Group > another CRA > HG.
Also inspect the hunt group, especially the members...make sure that other place calls go to isn't a HG member that only hits if all other members are busy...easy test: put all the HG members you know about in DND, call the ext of HG, see if something else rings.