Task in push queue not starting with Error Code 140

Question

Sometimes (seems randomly) when I schedule a task in a GAE push queue the task doesnt start and I receive this error message:

A problem was encountered with the process that handled this request, causing it to exit. 
This is likely to cause a new process to be used for the next request to your application. 
If you see this message frequently, you may be throwing exceptions during the initialization of your application. 
(Error code 140)

Despite the message "this is likely to cause a new process to be used for the nexr request" no retry is attempted and the task never executes. Anyone knows a possible reason? I cant figure out which error is code 140. (NB: not 104!)

Here is the task configuration in queue.xml:

<queue>
    <name>async-documents-storage-consistency-ops</name>
    <mode>push</mode>
    <rate>1/s</rate>
    <bucket-size>10</bucket-size>
    <max-concurrent-requests>10</max-concurrent-requests>
    <retry-parameters>
        <task-retry-limit>0</task-retry-limit>
    </retry-parameters>
</queue>