Some operations done on old files in Webtop throws exception after the Webtop has been customized

I am facing a problem in webtop after customization.

I am using webtop 6.5SP2 and JDk 1.5

I have imported some files in the webtop. Later I have customised the webtop with out deleting the old files. I have customised the edit propeties, import, checkin, cancel checkout and some more components. After I have customised the webtop, some of the functionality does not work for some of the old files which where present in webtop before customization, they throw exceptions for example- for edit properties it throws exception stating "failed to save properties". But the same functionalities work as expected if I do it on new files.

What is the reason for this problem? Could anyone explain the reason for this issue and how to overcome this issue.