I just wanted to add to this thread, as I came across it while looking for a possible solution.
My system was working absolutely fine, until I changed the english.php file (unfortunately, I had done several modifications at the same time, without feeling the need to recheck the shop). I'm not sure how but I must have done something. When I clicked on Add to Cart, Add Review or Checkout (from the shopping basket), I was sent to a blank page.
I ran the Debug tool, and got a headers already sent error for my languages/mytemplate/english.php.
I removed the english.php from includes/languages/mytemplate, so that it went back to use the default, and all works fine again.
I'm figuring I've deleted or added something somewhere in there, will go and see when I get the chance, but for now it's working, so it can stay that way until I get more time!
:clap: