Thread:Project:Support desk/Fatal error: Class 'MonoBookTemplate' not found in.../reply (8)

Ok, that means that you are not by accident using wrong files, but that you have the same files as you had before.

The class ModernTemplate is defined in SkinModern.php, not in Modern.php. Is it correct that you get the error for SkinModern.php and not for Modern.php?

Finally: On the new server, after you extracted your archive, do you then have a file skins/Monobook/MonoBookTemplate.php, which contains the class MonoBookTemplate?