It’s been quite a while now since it switched to Tap-a-Talk, so I don’t remember exactly, but it seems to me there was a point after the switch where Tap-a-Talk had a notice they were making me change my username & password to continue on the forum. Maybe you got that message and missed it, and now it’s just locking out the old account? Just throwing out a wild guess, I don’t have any other idea.
I rarely ever check there anymore, that platform is so annoying, but occasionally one of my FE friends lets me know there’s something interesting and I take a look. I just tried signing on and was successful. Sorry, I don’t have anything else helpful.