Classifieds error: "This field is required" in Content box

Please see screencast to see the problem happening:

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The problem: when placing a new or editing an existing Classified ad, the Content box throws an error when clciking "Save Changes" to post the ad. The error is highlighted below the Content box and says "This field is required." However if I simply click into that box, I can click "Save Changes" successfully without any other errors.

I found this unresolved thread that is similar to my problem:

NOTE: I've tested this with and without plugins activated and with default WP 2015 theme. No change, same issue.