I have just upgraded wordpress to version 4.7.3 on windows server 2008 R2. mysql is currently at version 5.1.72 and php was at version 5.6.
Everything worked fine.
I then added PHP 7.1.1 to IIS and changed to that version and now I get a 500 error on the web page.
I disabled all the plug ins and I still get the error. I reset the database password and I still get the error.
Is there anything else I can check/change/upgrade to resolve this?
After enabling debug mode I get the following line in the error file
PHP Notice: Undefined index: input in D:\Websites\MYWEBSITE\wp-includes\kses.php on line 866