"a web part of web form control on this page cannot be displayed or imported sharepoint 2010", It's a generic error in sharepoint 2010 that in the message it self they are explaining the problem that it was not registered as safe. I was googled for some time to get the exact details and resolution for that error i have found an excellent article here
http://www.bluedoglimited.com/SharePointThoughts/Lists/Posts/Post.aspx?ID=189.
It will applicable for not only sharepoint 2010, also reflects Sharepoint all versions to explain the error details.
http://www.bluedoglimited.com/SharePointThoughts/Lists/Posts/Post.aspx?ID=189.
It will applicable for not only sharepoint 2010, also reflects Sharepoint all versions to explain the error details.