I tried to enter as solo FP assault on Tython as imperial agent ( lvl 70) and sith warrior (lvl 55), but neither worked. Therefore I can't do Forged Alliance at all, which is annoying. I have read that it's a known bug and many can't enter that certain Flashpoint. I use the European server Darth Malgus.
EDIT: It doesn't work on smuggler class either.