MYBB: How to Fix: "Authorization code mismatch" issue in Mybb Forum

Original Post made by Shade at #MyBB (I take NO credit for this fix)
If you have upgraded to 1.8.16 or above, please read the following instructions carefully.
#Templates requiring changes:
error_nopermission
header_welcomeblock_guest
member_login
portal_welcome_guesttext
1. Localize form tag
Open each template and look for <form
action="member.php" *> . They are the forms you use when logging in. The form tag will likely include other
attributes, which you can ignore.
2. Add token
Add after <form action="member.php"> the following
piece of code:
Code:
<input name="my_post_key" type="hidden" value="{$
mybb->post_code}" />
None
Select All
It's that simple. Your users will now be able to login
again. my_post_key input is in fact now mandatory to
be sent alongside other form parameters in order to
reduce potential malicious attacks.
Note that upgrade scripts will revert naïve templates
and will handle the change automatically. This guide
is aimed at all who have custom themes with these
templates customized.