I've been using these alerts for the better part of a year without issue. I'm not sure why this happened.

Answer:  We've seen this happen when an Admin has inadvertently done one or more of the following:

  1.   Removed the field that is being updated from the page layout
  2.   Deleted the field our Workflow App is updating 
  3.   Added a new required field or validation rule, which is preventing our App from updating your "field update" value (You can test this my creating a list view that contains the update field. Then see if you are able to manually update that field.
    1. Even if you've created a new Validation Rule that has nothing to do with your "field update' field, this can prevent our App from updating the "Field Update" value, because you have records that already to not meet your new validation rule requirement. These records did not meet the rule at the time the rule was created. So now that our App is trying to update the record, the validation rule applies, and Salesforce will reject the field update until the record meets the rule requirement. Do a mass update to the fields required in the validation rule and that should solve your problem. Or turn off the validation rule. We have another app called "Co Pilot" that is much more useful that validation rule. Go here to read about Co Pilot and or call us at 480/696-3812 to get a demo how of how this is better than writing validation rule: http://www.crmguidance.com/salesforce-helpco-pilot