This seems odd. My iPad logs into this discussion website(discussions.agilebits.com) fine, but my Mac does not. I have my Mac and my iPad synced thru Dropbox. They both have identical User ID, PW, website, etc. In fact, I created the login item on my Mac and my iPad obtained this info from Dropbox sync.
When I try to login with 1P from my Mac, I am taken to the site, but nothing happens. I do have "Keep signed in" check marked. If I manually login to your discussion forum, I can paste User ID and PW from iP and it works fine, but it will not log in automatically.