Here is some feedback! 
It looks good. I think it kind of misses some visual identity, though. 
. (EDIT: I get definitively the same feeling as Penar about the identity)
Mobile
It looks good in portrait mode.
Some issues in landscape mode:
- The header is cut off. You can’t scroll up, but you can scroll down.
Screenshot
It happens in portrait mode, too.
The keyboard pushes content up on the login page. It doesn’t happen on the register page.
About the landscape mode. Most people won’t use it, but it can happen, and it’s not a great user experience. I think you can adjust a few CSS to make the blocks float around the welcome message. Not ideal, but it’s okay:
However, the most visually appealing to me is to split credentials and social buttons in half:
- Not directly related. On mobile, the first input is focused, and It opens the keyboard directly. (I don’t know if it’s the same behavior everywhere)
I think it worsens the user experience for two reasons:- you don’t see the full page at first, and it’s not welcoming.
- you don’t see all the login options. (especially for users using social login – you have to close the keyboard every time)
Desktop
It mostly looks good, with some issues.
- It looks good on medium screen size:
- On large screen, I’m not sure. It looks good; however, while having all the social buttons on one line seems convenient, it looks unusual (small content centered, with below a large content). It’s not wrong, and it looks okay – it might be a habit from the previous style as your attention is focused on one centered place.
What you did before was actually quite elegant, and as suggested Penar, a max-width would be enough to make it look good. I would not mind seeing two columns on a large screen, then it wraps below automatically under some limit.
- On the responsive side – under some minimum height, you get a scrollbar on the left panel. I can’t test on a real device if it happens too. I think It would be more natural for user to scroll the page instead of the internal component.
Common
- The forum scrollbar is apparent when the login modal is opened, and you can play with it. It might confuses the user into thinking there is something when there is nothing. It can be trippy if the modal scrollbar appears as well at the same time.