I guess you have already checked that the path is correct several times and that you have deployed all DLL’s and ASP.NET files. What is wrong?

LoadControl() parses and compiles your markup

A quick look with reflection reveals the LoadControl() is called to verify that the web part really exists. When you ask ASP.NET for a user control it will parse the ASCX-file, generate code and then compile it.

So there is a lot that can go wrong and all exceptions from this process is swallowed by the user interface and replaced with virtual path could not be loaded.

A good way to actually see the exception is to create a dummy aspx file and register all user controls and reference them from markup. When you try to view the aspx file any compilation error from the user controls will surface.

Examine compile and pages tag in your web.config

The most common reason markup works fine on one machine but not another is differences in web.config. Check the compilation and pages tag and compare them for differences between the machines.