Symptoms
PlanParamsGetAndValidate_API returns corrupted xml result.
Cause
PlanParamsGetAndValidate_API uses checkServiceParams method. Backspace sign (^H, "\010") is returned by checkServiceParams method (if domain is already registered). Output XML file is corrupted because of ^H sign, which is a bug with id #PBA-54198 ("The backspace sign is not escaped in the API method XML response").
Resolution
Update to PBA 5.5.4 or above.