Custom Field does not delete after just being added

[pirom]

Located within edit-pages & edit-posts there are Custom Fields which I have just added. After adding these Custom Fields if I were to delete it, the red ajax animation would get stuck and a "Something strange happened. Try refreshing the page." error would appear. This Custom Field does not get delete as it still remains after refreshing the page.

Deletion works on other Custom Fields, just not the one that you've just created.
If you create a Custom Field, and perform other actions such as "Save & Continue Editing" then deletion of last Custom Field also works.

SUMMARY:

1) Create custom field within page or post

2) Delete that custom field you've just created

3) Error Occurs...

[Nazgul]

Duplicate of #4474.