Error initializing activity MailSource

Error:

We have a process flow triggered by a mail trigger that fails intermittently, apparently on the trigger.

Invariably, the diagnostic message reads,

"Error initializing activity MailSource:<some mail trigger name>:0101000001091
31791171313500054[String index out of range: -1]".

Cause:

You are attempting to trigger a flow without setting eventContextEnabled set to true

Solution:

Confirm that are you trying to pass the data from this mail trigger in the process flow. If yes, then eventContextEnabled property is set to true

http://support.adeptia.com/entries/21287846-When-to-use-eventContextEnabled-