How to deal with the "ErrorWebPart" error displayed in the web part maintenance page

When a web part is deployed into production server, the web part displays and works properly, but shows a "ErrorWebPart" error in the web part maintenance page under the Type and "Error" under the title. In the meanwhile, you can't edit the web page in FrontPage 2003 once the web part in installed as it tells the web part is not safe.

Microsoft KB Article 830342 will help you fix this problem.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;830342

The following methods extract from above KB.
Method 1: Modify the Internet Protocol Address Assignment.
If you have configured the virtual server with an assigned Internet Protocol (IP) address in Internet Information Services (IIS) Manager, configure the virtual server to use the (All Unassigned) IP address option.

Method 2: Move Assemblies to the Global Assembly Cache
If you host a virtual server that has a host header name and you install your assemblies to the Bin folder, move the assemblies to the global assembly cache.