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