Task queue operations may rarely generate a TransientFailureException, which will look similar to this:
com.google.appengine.api.taskqueue.TransientFailureException:
at com.google.appengine.api.taskqueue.QueueApiHelper.translateError(QueueApiHelper.java:106)
at com.google.appengine.api.taskqueue.QueueApiHelper.translateError(QueueApiHelper.java:153)
In general this exception indicates a temporary issue with the underlying App Engine infrastructure, not a failure of the application code. This problem should fix itself shortly.
To mitigate this exception, catch it with a try/catch block and attempt to requeue the task. If that fails, insert a delay of a few seconds before attempting to requeue the task. Redeploying the application may also help.