Custom Fields No Longer working

When I am writing a post and want to add custom fields, I click “Add Custom Field” and it does not work.

What I find troublesome is that it used to work without a problem – and then just stopped. (I didn’t make any changes that would have prompted it). I have keys that I was using. I choose the key, type in the value, click “Add Custom Field” and it does nothing.

I upgraded to the latest version today, hoping that would fix the problem, but no.

Any help is much appreciated.