I'm finding no helpful information on this.
When an error mentions a 301-prefixed Id, I know it's Flow related (probably Process Builder), but I have no clue how to hack a URL to showing me the Flow (or Process Builder) in question. Typical http-salesforceDOTcom/ID references do not work.
I have no clue how to debug such errors without the ability to know WHAT Flow / Process Builder routine it means. :-(
In the screenshot, 301* is the "Flow", while a5A is a managed Custom object (Customer Master from Rootstock). I can't simply put the 301* reference after a Salesforce URL like most objects.
Insufficient Privileges You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary. For more information, see Insufficient Privileges Errors.
UPDATE 1: This is the closest I came to a solution - someone mentioning the same concern in an Idea: https://success.salesforce.com/ideaView?id=08730000000DmI7AAK - a year ago.