Post update issue with metadata

[marko-srb]

Here's an exact preview (attached video) of the behavior / error message that I get when I try to save some post edits. This happened only on certain posts/sites, and I haven't have this issue/message on other sites/posts. List of sites I had this issue with:

https://wordpress.org/showcase-v2/microsoft-news-center/
https://wordpress.org/showcase-v2/the-village-voice/
https://wordpress.org/showcase-v2/global-news/
https://wordpress.org/showcase-v2/omg-ubuntu/
https://wordpress.org/showcase-v2/danielle-steel/
https://wordpress.org/showcase-v2/the-swell-season/
https://wordpress.org/showcase-v2/childrens-rights/
https://wordpress.org/showcase-v2/whatever/
https://wordpress.org/showcase-v2/playstationblog/
https://wordpress.org/showcase-v2/die-tageszeitung/

update-post-issue-metadata.mov

Textual description of the issue: an error message shows when I try to update certain posts, saying metadata couldn't be updated.

CC: @jasmussen