I get a similar effect with my tamron adaptall 90mm f2.5 macro, obviously a very different beast, but I understand it's due to the rear element being flat. A lens hood sorts it out, but I wonder if you are experiencing something similar, along with a similar issue I observed with using extension tubes on certain lenses.
I think Bryn is right and you are getting light bounced directly back into the lens (as I understand it it's the sensor itself that is being reflected back, which is why it appears purple), maybe bouncing back from your reflective subject. I would suggest the following:
- It looks like you are using a twin flash (MT24?) Can these be angled more to try to prevent light bouncing directly back?
- Make a small lens hood of some kind (rolled up bit of card should do it) - if the light is being reflected directly back through the lens, I don't understand how this works; but it works for me, so worth a shot!
The ping pong ball is a good idea anyway, I've seen excellent results with that as the diffuser (even if it does look a bit fiddly).
Good luck! It seems like you are going to get some amazing results once you've overcome this little obstacle.