When breaking role inheritance on the out of the box Workflow Task List (2013), a custom workflow is suspended when assigning a task(System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary. ).
As soon as task list is inheriting permissions again, the workflow is able to assign tasks. (Similar issue: http://social.msdn.microsoft.com/Forums/en-US/8c60da76-8299-436c-8528-802fa5a594c3/2013-workflow-with-unique-permissions-on-task-list-ends-up-suspended).
I have not found a solution to this issue so far.