Look For some theme setting if available, can you share the site url.
You will not be able to fix this from the WordPress admin dashboard, as you mentioned it is redirecting you even when you login. So you will have to access your code using FTP clients like Filezila or if you have access to Cpanel that would also help.
Are you aware who is your hosting provider? where all the code is placed.
Your theme may have code which is generating this error, it is not necessary that the file which is mentioned is the cause of the error, if the update is not breaking anything you can disable the error reporting in your wp-config.php file.
https://wordpress.stackexchange.com/questions/19689/wp-debug-is-not-set-but-im-still-getting-warnings.
