we developed some new master pages for SharePoint site in farm sever
installation,and after 6 months i added new mater page with new pages
inherits from it, and it was working fine on test machine, but after
moving "master page & pages" to production, there is no way to Add or
edit any webpart from browser for any webpart, and when i came back to
test machine i found the same error saying : {Cannot save the property
settings for this Web Part. Exception occurred. (Exception from
HRESULT: 0x80020009 (DISP_E_EXCEPTION)) },
so i used sharepoint designer for adding new smart parts and give path
to my User Control from there,
and the same thing happened later in the Test Server

the amazing thing is when i create new page inherits from the Default
master page that shipped with the sharepoint, Everything is solved and
you can use all the activities "Edit & Add & .." through the browser,

I cannot find what the bug that makes the master pages gives such error.