This error almost drove me over the edge... The SharePoint properties pane in Visual Studio would show the error "An error occured trying to load the page. Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)"

After pulling much of my hair out (And doing some trial & error testing) I realized that the problem was that the pane was looking to see if there were any new web parts added to the project each time it loaded. I had added an abstract WebPart base class which some of my other WebParts would inherit from. Problem was this abstract WebPart class was not decorated with the Guid attribute, which is evidently required by the SharePoint property page. I added the Guid attribute and the error goes away.

The only problem is this WebPart base class is now included in the SharePoint Solution as a feature. I haven't found a way as of yet to exclude it.