First make sure to reinstall the latest versions of UVI Workstation and your piano(s) on your system, 



If it does not help you are most likely experiencing this issue because you have re-ordered or removed scripted effects  (i.e. used in the instrument's GUIor script) from UVI Workstation's FX section.


Instead of reordering or removing these scripted effects, simply bypass them and you will not encounter this issue.