Had the same problem today. I was able to resolve by editing boot.php under /plugins/insert-php/admin/boot.php and remove lines 81 to 95.
This allowed me to navigate the admin area. I really do enjoy this plugin and have been using it for many years. After the latest update, all my [insert_php] short codes stopped working and my custom php code stopped working and this about page kept coming up.
I went back to the older version of Insert PHP and it was still redirecting to the about page, but did fix my custom php code.
So after some time looking through the code, I found where it was redirecting in the boot.ini.
All good to go.
Always fun updating plugins.. Like pulling teeth…
