A 'rudder lock'?
I can only vaguely remember hearing the term. Damned if I can remember what it is!
It sounds like an aerodynamic effect caused by too much sideslip? Is it related to a fin stall?
I'll take a punt at an answer. I imagine that the solution would be to change IAS - probably a reduction - to reduce the magnitude of the aerodynamic forces, or to roll to induce a sideslip to change the airflow pattern.