Running Sitefinity 3.7 Standard Edition
I have some textareas on a normal page in a Generic Content control to hold some code for links for users to copy and paste from. This was causing the same error in the admin when the page was being published:
A potentially dangerous Request.Form value was detected from the client
My "Sitefinity/Admin/CtrlProps.aspx" page already had validateRequest="false" set on it. I had to update "/Sitefinity/Admin/EditPage.aspx" to fix it.
Now it works.