There are two problems:
1: One of the projects (MediaPortal.UI) isn't being loaded correctly because of some .Net versioning issue. Googling indicates it's something to do with a dependency on an assembly compiled with .Net 2. Could you check that the project is set to compile to .Net 3.5 (3 might work to)?
2: the second problem is the one that finally stops execution since it can't load the skin. But can't see how this error could ever occur during normal execution so I would guess it's caused by problem 1.