I tried this (my suggestion below) and am now waiting/hoping the problem with not being able to enter my password into the box after screensaver is gone for good. I'm hopeful as I've read of others (in various versions of Ubuntu) who have achieved success doing this. It makes sense to me too.
From the command prompt in your home directory, list all files including hidden files:
ls -lsagF
If your results include ANYTHING owned by root, change it to you (your user name) owing it. This is often the case with .Xauthority which is often 'the' problem.
Error results looks like this (note root is the owner):
-rw------- 1 root 220 Feb 1 20:24 .Xauthority
It SHOULD look like this (if your username is 'fred'):
-rw------- 1 fred 220 Feb 1 19:41 .Xauthority
The fix (assuming your user name is "fred") is:
sudo chown fred:fred .Xauthority
Do the same with any other file or folder in your home directory.
Check again w/
ls -lsagF
until all are owned by you (the fix looks like this):
-rw------- 1 fred 220 Feb 1 19:41 .Xauthority
This MAY fix your inability to login after screensaver issue.
I'm still waiting to see if mine messes up again.
Open Disclosure: This is NOT my fix - it was recommend by another; and now that I know what to search for, I see discussion on it being successful over the last 9 years in various versions of Ubuntu AND others.
This 2014 article in AskUbuntu discusses root NOT owning .Xauthority also.
Why .Xauthority needs not to be owned by the root account in order to get past login?
I hope this is working for me now (fingers still crossed) and hope it works for you.
