Suddenly, We are not able to edit and Channel Fields. We are using: EE v2.8.1 - Build Date: 20140314
If we go to: Admin > Channel Administration > Channel Fields > Example Group > Slider Image and not even change any values but simply hit "Update" button
its giving error saying: disallowed key character
We did some google and we tried all different solutions: clearing Cookies, Cookie Prefix but nothing is helping
any help or pointers to solve this issue will be of great help. Also same error comes up if we edit any template and if that template has a word like name="fullname"
see screenshot attached for the page we are hitting update that gives us error.