While trying to open a form for customization for a custom entity we were getting below error within the maker portal.
The user was an admin user.
The same was the case in both Chrome and Edge browsers.
The only way we could proceed was by switching to classic experience.
Switching to Classic worked
After updating the form in classic and publishing the changes, the form started working properly in the maker portal also.
We were able to customize the form in both Chrome and Edge.
Hope it helps..