I have a custom web part developed in Sharepoint 2003, which I am
deploying using custom policy file instead of installing the dll in
GAC.

I have given the dll 'FullTrust' permission in codeGroup and placed
the codeGroup before Asp.net entry. All the other permissions in my
custom policy file are similar to wss_minimal.

The custom web part when deployed displays an error
"One of the properties of the Web Part has an incorrect format.
Windows SharePoint Services cannot deserialize the Web Part. Check
the format of the properties and try again"

But executes fine if I install the dll in GAC. Does FullTrust not
grant full permission to that dll.

Have any one faced similar problem before?