I just downloaded and installed the trial version of Malwarebytes and had the same issue as the OP. I found various other posts that offered no solutions and apparently abandoned by their respective OP's as they didn't even answer the initial responses.
In my case I did try to log into Windows 10 safe mode, but was again unable to log into my account and was given a temporary account which proved useless to even find the installation of malwarebytes. So I re-started my machine and again got the "cannot sign into account" message. However after dismissing it I was able to find the malwarebytes program. Upon starting it, I did as Porthos instructed in a previous post. After restarting again, I was able to log into my normal account as if nothing had happened.
So now the question becomes several:
Do I turn the real-time protection back on?
Can I let malwarebytes run at startup?
Do I even want to consider the premium version of Malwarebytes?