In case of failed Validation on Create Issue, the following exception is raised :
com.atlassian.jira.exception.IssueNotFoundException: Issue with id 'null' or key 'null' could not be found in the system

• Why Minyaa overrides the Create Issue action :
  When an error was raised by a Validator, JIRA (before 4.1.x) shows only that there was an error, without giving the details and a focus on concerned field.
  Override consists in catching CreateException, and look recursively into original Exception in order to identify InvalidInputException

• Root Cause for your current Issue :
  Since JIRA 4.1.x, the CreateException is catched befaore Minyaa overide and replaced by a IssueResult.
  Since the exception is no more raised, the override done Minyaa try to complete the Issue Creation, but the context is no more consistant to do it (due to the validation error )

Workaround : You can disable the Minyaa Override as follow ...
Go to System / Plugin administration page
Select Minyaa JIRA Plugin for Workflows Management
Disable Actions for CreateIssue (CreateIssueActions)