Getimo,
The changes in 4.0.1 shouldn't have affected registration or login at all. Almost all changes were in the JFBCContent plugin (/plugins/content/jfbccontent.php).
As for the problems you're seeing, there's nothing I can see that should be causing the login problems. When I click the button, I'm successfully redirected to the component/jfbconnect/?view=loginregister page as expected. I didn't register an account, but I gladly can if you'd like. This was with Chrome on Windows. Can you try a different browser, or let us know what actually is happening when you try to log in? Pop up doesn't go away, doesn't show the login form, etc.
For K2 Like's going slower, that seems to be a Facebook issue right now. We're noticing them a little slower on a lot of sites (not necessarily JFBConnect, or even Joomla ones).
Yes, the switch to the K2 URLs isn't nice, but as you said, there isn't much of a better way. The reason we use the SEF URL is because that should be the best/final URL. If we use the original, non-SEF Joomla one, that can change a lot depending on ordering of parameters (which we can fix), as well as it may include an Itemid and other things that we can't correct for. Due to the recent release of 3.4 which had our initial K2 support less than a month ago, we made the decision to just fix the problem instead of implementing some weird work around that would work forward and back. It is unfortunate, and we apologize for the inconvenience.
For the suggestions:
* We agree with the selection of social buttons. That will be in a future release. If you want to always have one removed, we can point you to the line(s) of code to alter to do that (pretty easy).
* Changelog of altered files is difficult. This probably won't be done going forward as it's never been a common problem with bugs we have (and we're proactive about fixing what is found).
* A newsletter announcement for the 4.0 release is going out tomorrow. We always announce the minor (x.y) releases, but not always the bug-fix versions, as usually those fixes don't affect all users. We don't like going overboard with emails as any of our plugins, modules, or the main component does have updates regularly... it would get annoying fast
Finally, one thing I did notice was that on the Login/Register page, the welcome text is instead "COM_JFBCONNECT_THANKS_FOR_SIGNING_IN". You may want to check your com_jfbconnect.ini language file for that string. We'll also check to make sure it's in the German version we distribute.
Keep us posted on anything you find,
Alex