Users hit blank white page upon clicking Email Confirmation Link - Signup Broken?!

Hi all!

Hoping someone may have some recommendations on how to fix this issue.

Our network seems to have hit a snag with the user signup process.
Currently, when a user registers for an account, they receive an email confirmation email like they should.

However, when they CLICK the activation link, they hit a blank white page. If you reload the page or try to click the link AGAIN, you get sent to a page that says user is already activated, and it has a little box underneath that show a place to manually enter the activation code.

So although a bit buggy, it would appear that at least the user is now "activated". However, AFTER going through that process, if you try to log in with that user account, you get an Invalid Password error.

So then, if you do a password reset form, click on THAT link in your email, and reset your password (even to the same one you used originally), ONLY then can you actually log into the site.

A very, very broken and buggy process, which is seriously stunting our growth right now (we are getting a large number of emails from people saying they cannot sign up. Who knows HOW many more people just didn't bother emailing us with the issue?!)

We've tried going through and eliminating conflicting plugins. Of course we could have overlooked something, but I was hoping perhaps someone has seen this issue before and can point us in the right direction to start looking.

This support forum is amazing! Thank you all so much for taking the time to read this and respond.