The main menu will not work unless you launch the "Trackmania United Forever Settings", click "Configure", click "Advanced", and in the Display tab, make sure that Shader Quality is set to PC1 (or PC0). Anything above PC1 will not render the main menu for me, it will just show a spinning globe.

What I did is made sure the Customise checkbox was checked, then selected the Preset "VeryHigh Quality", then set it back to "None". When you set it to None, it allows you to freely select settings. Then I switched Shader Quality to PC1. Otherwise, I cranked everything else up as high as it would go and it runs buttery smooth.