edited from support email

Thanks for your reply.
We tried re-installing all the available versions of DirectRT and none of them worked; hoverer, we found a possible suspect - we recently installed new display cards, in order to set the screen's refresh rate to exactly 100 MHz. When we uninstalled the new card's driver, DirectRT worked again.

We really need to be able to set that refresh rate and need an urgent solution. In case it matters, the cards are Shafir's HD 4350 256MB, and the processor of the cards is ATI's.