I haven't experienced this problem myself, but there seem to be some people get this exception when they use Son of SmartPart to load ASP.NET 2.0 user controls in combination with the Web Deployment Projects add-in. Steve Hartzog blogged about the solution for this problem: add the WebResource.axd to the excluded paths in SharePoint Central Administration. If you want to know how and why, check one of my previous posts. If you just want to get it fixed, below is the summary. I'm going to include this step in the Installation Guide of the Son of SmartPart, once again thanks Steve for bringing this to my attention!
On your SharePoint server, open up the SharePoint Central Administration site from the Administrative Tools start menu group and click the “Configure virtual server settings” link. On the next page you will see a list of all your virtual servers, click on the one that you would like to configure (you have to repeat the steps for each virtual server). Next, click the “Define managed paths” link. In the “Add a New Path” section, fill out WebResource.axd as the name of the path, for the type select “Exclude path”. Finally click the OK button. Now you can use WebResource in SharePoint sites!