I just upgraded to 1.4.3 and am also having this problem...in this way.
The registration form - if the user ever hits the return key, send the user to the front page of the site (in my case not the forum) and does away with any data they have entered. If they carefully tab through it, it might take.
Here's what I have done.
Flushed all cache, history, etc.
removed the old add-on's which may have affected the login form (mandatory location, reg form time) - and checked template mods - they do not show up.
Uploaded new copies of the two Register.php copies in the XF upgrade package.
It's still happening. I have very few add-ons - turned them off to check and it still happens.
Any suggestions on what to do next?
Example of this behavior - if I use tab and go through the reg. form to the end - but then place my mouse back in the PW field and click, that triggers it. So does hitting return or enter in certain fields (not all).
Hmmm....just noticed that the article (link) on the notice at the top of the screen is where they are being redirected to........I'll turn off notices....I already turned off all add-ons, but that's not an add-on