is there any custom .NET code installed, ie 3rd party or your own custom features?
its weird the way a skin change can cause this kind of error unless it includes 3rd party controls
from more googling about the error I found among other things this post, which sounds like there can be problems if the build machine ie my local dev machine is trying to target the .NET 4 framework but it has .NET 4.5 installed, theoretically, it should not be a problem but in practice it apparently can be a problem, because it sounds like even though VS asked for .NET 4 for compiling it got 4.5. so possibly the dlls I compiled for .NET 4 for mojoportal are not really compatible with .NET 4 or else some other dll from a 3rd party or custom code might not be.
I don't have any machines that have not been updated to 4.5 and I don't know for sure if my dev machine is affected in a way that things I compile for .NET 4 in VS are actually targeting 4.5. I know that I have had 4.5 on this machine ever since it came out 2 years ago and we have had multiple releases that only targeted .NET 4 in that time and only the most recent version of mojoPortal after 2 years of waiting we finally changed our main target to 4.5 but we still kept alternate versions of the .csproj files to target .NET 4 so that we could continue providing dlls for .NET 4. So I certainly would have expected to know long before now if my machine was not compiling correctly when the build was targeting .NET 4. So, I m not saying for sure there is something wrong with our .NET 4 dlls but these are weird errors you are having that point to errors that look like framework mismatch errors.
in any case the best way forward in my opinion is to change to 4.5 hosting, any host has this on newer machines and your current host can probably move your site to a newer machine and you will have much less headaches I think.