I am having the same issues. I forget specifically when it broke, but my site w/ register-plus was working great in 2.8.x and now in 2.9.x it stopped working.
those features all worked in 2.8 wordpress, and I emailed the developer twice so far and have not heard back. I would really like to hear back from them and find out if this will be updated, but I haven’t heard back from them yet.
try disabling the visual editor mode.
it looks like the link-harvest plugin is not working properly. disable that plugin through your plugins panel and see if you can post then.
