I also have this problem.
Popup the "illegal file format" error msg.
Can't find /sprite/npc/4_m_jpnoji2.act and /sprite/npc/4_m_jpnoji2.spr.
My data.ini read hercules.grf first.
I think the 4_m_jpnoji2.act and 4_m_jpnoji2.spr in hercules.grf have some error. And I use GRF Editor open hercules.grf find those files.
But I don't know how to fix this error.
I just delete the files.
And it can work now!