The 'Assignee Only Condition' fails if assignee is changed on transition screen.
This is due to the fact that the issue is retrieved from the database initially and then cached. This cached issue is subsequently used throughout the conidtion and results in errors when validating - workaround is to retrieve issue from database each time for validation purposes.
Please replace the original class with the attached class:
../java/com/atlassian/jira/workflow/condition/AbstractJiraCondition.java