In my case, I had a Person Picker field with invalid values. The person had left our organization. Once I removed their name, the record would update.

In order to find the record causing the issue, I created a simple update query. It selected the record ID, and updated a dummy field with filler text. When the error message appeared (ie. the dummy field would not take the filler text), the ID at the top of the Table View was the record that was throwing the error.