Yes - you need to check your code - one of your properties probably has
a wrong type or some such thing - for example you may have declared an
integer but are trying to store a string. Basically SharePoint cannot
convert the properties stored in the Web Part to useable values.

But am still facing the same problem.
What you are saying is right.

I have default property defined as "Text", but want to display charts
(Statistics) on the web  part.
when I declare a chart it starts giving this error in sharepoint.
what Default Property do I have to give for displaying charts?
Am using TeeChart for .net assembly for the same.

Any inputs on the same are valuable.

I saw this error once. It was related to permission to the windows temp
dir. You should strongly name your assembly too.
If the webpart access file directories u may need to change the security level
in web.config.