hmm..I get into them perfectly fine.
Have the people that are having trouble delete all their temporary internet files. I notice you have no right blocks showing in the Forums. I dunno if it is possible that they still have the temp. internet files from when your site did not have the right blocks removed from the forums..who know.
IF that does not work. I would try putting the right blocks back and see if that fixes it. Try anything at this point...
HMM or are you running the STANDARD phpBB Forums, not the ported ones from Tom's Site?
mikem