Hi, have a pretty simple and straightforward Lead alert process.  I have a Process Builder "Flow" that is initiated every time a new Lead is created, and as a next step sets a 12 hour timer to initiate an invocable process.  

The invocable process checks to see if the Lead has not been assigned (pulled out of the Web Lead Queue, the default 'owner' of the Web Leads) and if it is still "Unread by Owner = True".  This Flow worked for a few weeks but now is no longer working even though nothing changed on our end.  Below is the flow error.  Any help is appreciated!  Thanks everyone!

This report lists the elements that the flow interview executed. The report is a beta feature.
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Flow Details
Flow Name: Lead_is_Created
Type: Record Change Process
Version: 3
Status: Active
Flow Interview Details
Interview Label: Lead_is_Created-3_InterviewLabel
Current User: Bobbi McConnell (005d0000000cERN)
Start time: 2/21/2018 3:55 PM
Duration: 43,174 seconds
How the Interview Started
Bobbi Smith (005d0000000cERN) started the flow interview.
Some of this flow's variables were set when the interview started.
myVariable_old = null
myVariable_current = 00Q0V00000y2bzHUAQ
ASSIGNMENT: myVariable_waitStartTimeAssignment
{!myVariable_waitStartTimeVariable} Equals {!Flow.CurrentDateTime}
Result
{!myVariable_waitStartTimeVariable} = "2/21/2018 3:55 PM"
DECISION: myPreWaitDecision_myWait_myRule_1
Executed this outcome: myPreWaitRule_myWait_myRule_1
Outcome conditions: and
1. {!formula_myRule_1} (true) Equals true
Logic: All conditions must be true (AND)
DECISION: myDecision
Executed this outcome: myRule_1
Outcome conditions: and
1. {!formula_myRule_1} (true) Equals true
Logic: All conditions must be true (AND)
WAIT: myWait_myRule_1
The waiting conditions for the following wait events were met.
Wait Event: myWaitEvent_myWait_myRule_1_event_0
Waiting Conditions:
1. {!myWaitEvent_myWait_myRule_1_event_0_postActionExecutionVariable} (false) Equals false
Logic: All conditions must be true (AND)

Event Type: Alarm: Relative Time
Object Type = Lead
Record ID = {!myVariable_current.Id} (00Q0V00000y2bzHUAQ)
Offset Unit = Hours
Offset Number = 12
Base Date/Time Field = CreatedDate
Result
Started waiting at 2/21/2018 3:55 PM. The interview started waiting at 2/21/2018 3:55 PM.
The interview was resumed at 2/22/2018 3:55 AM by Bobbi Smith (005d0000000cERN). WAIT: myWait_myRule_1
Wait event myWaitEvent_myWait_myRule_1_event_0 occurred.
The following values from the event were assigned to flow variables.
None.
DECISION: myPostWaitDecision_myWaitEvent_myWait_myRule_1_event_0
Executed this outcome: myPostWaitRule_myWaitEvent_myWait_myRule_1_event_0
Outcome conditions: and
1. {!myVariable_current.CreatedDate} (2/21/2018 3:55 PM) Is null false
Logic: All conditions must be true (AND)
LEAD_STALE_LEAD_ALERT (FLOWS): myWaitEvent_myWait_myRule_1_event_0_SA1
Inputs:
SObject = {!myVariable_current} (00Q0V00000y2bzHUAQ) Error Occurred: An error occurred when trying to start a flow interview. Salesforce Error ID: 1400152300-1998 (265186732)