Error “Internal Server Error” in checkout page

Have you got any screen shots or url we can try, I had something similar the other week but dont want to give you duff info.

Have you checked for conflict yet? I wrote a help article on how to check for conflict and similar common issue scenarios here: http://conschneider.de/how-to-check-and-resolve-conflict-in-wordpress-and-woocommerce/ – it includes a checklist which you might find helpful.