Hi,
I have this exact same issue. I do not have anything inside my child theme and I also get the exact same message:
Warning : include(template-blank-5.php): failed to open stream: No such file or directory in * /wp-includes/template-loader.php on line 106
Warning : include(): Failed opening ‘template-blank-5.php’ for inclusion (include_path=’.:/opt/cpanel/ea-php74/root/usr/share/pear’) in * /wp-includes/template-loader.php on line 106
This is clearly a theme update issue as when I reinstall a backup of the theme from my server, the issue goes away. Furthermore, the issue is only appearing on the login page and no other area so the site will appear normal until you try to login. That is when it throws the error.
Please advise on this as I too rely on Theme My Login to allow users to login to my site so this issue with the theme update not working needs to be solved.
Thank you.