PDO datetime exception when saving a Computed Field (but no datetime in my query)
In the end, something in the BOA or Entity update affected fetchField(). I resolved it by explicitly setting the index like so: fetchField(0).

I added a comment to api.drupal.org to hopefully help the next person who has the problem.

https://api.drupal.org/api/drupal/includes%21database%21database.inc/function/DatabaseStatementBase%3A%3AfetchField/7