I now have the same issue. After forgoing flashpoints in order to hit level 50...I got back and try to do the normal ones to unlock the hardmodes and I find I can't enter. That is not cool Bioware!
What does a level 50 have without Hardmodes? Not much to do is the answer. I don't want to put all the time and effort into another character because he might have the same issue when he hits a level I want to do flashpoints at.
I was able to do BT at level 12, but now at level 50 I can't do a single flashpoint regardless of group, non group etc. It's kind of frusterating/and infuriating.