if project name and project file name differ, it is not possible to rename the project, if the new name is the same as the project file name. - http://community.sharpdevelop.net/forums/t/15800.aspx

src/Main/Base/Project/Src/Gui/Pads/ProjectBrowser/TreeNodes/ProjectNode.cs

@@ -192,6 +192,15 @@ namespace ICSharpCode.SharpDevelop.Project
 //			}
 			string newFileName = Path.Combine(project.Directory, newName + Path.GetExtension(project.FileName));
 			
+			// see issue #2 on http://community.sharpdevelop.net/forums/t/15800.aspx:
+			// The name of the project and the file name might differ. So if the FileName is
+			// already the same as the new project file name, just update the name in the solution.
+			if (FileUtility.IsEqualFileName(newFileName, project.FileName)) {
+				project.Name = newName;
+				ProjectService.SaveSolution();
+				return;
+			}
+			
 			if (!FileService.RenameFile(project.FileName, newFileName, false)) {
 				return;
 			}