Custom Fields Not Updating/Saving when Editing Post

I am having problems with one of my custom fields not staying set when I go to edit a post. The field is “image” and I am posting a link to a photo there – so pretty basic stuff I’ve done hundreds of times before.

When I went to edit the post, I added the custom field and it shows as being there and the box flashes yellow as if it’s updated. I check the page and the image is showing all fine and wonderful. But when I go to change the text in the actual post itself and hit “Update” – now my custom field for “image” is blank and the picture is missing on the post page. I paste the link for the image back in and hit update in the custom field box – it flashes yellow and I check the post and it is there – but if I go and hit update again for the actual updating of the post it disappears again.

This is not happening with any of my other custom fields – just that one.

Any ideas or advice? Already tried unsuccessfully with no plugins.