I installed a theme in Magento 1 that is just working with onepage checkout. In general that is fine, but somehow it is not working. First, I type in my billing address/shipping address, but it stops and then it redirects me to the cart page.
500 (Internal Server Error)
So, I was searching for the saveBilling Method in
/app/code/core/Mage/Checkout/Type/onepage.php. But this function works fine and returns no error.
Any idea what next steps I could do to find that error? is it possible to extend that Method maybe? If yes, how can I look up for it?