Hi @forumfred
This error is likely a result of a failed attempt to execute a CloudCode.callTask() in your app runtime. CloudCode.callTask() will fail if the user is offline (naturally) but also if the task is not enabled or if App Triggers are not allowed for the task in the Task Settings.
In your case the likely cause is just users being offline. So you either need to wrap the call in a try catch loop, or first check if the user is online before attempting the call.