It really sounds like you have a bad file or files on the server ...
The point being, you should not get the 500 server error message or any bad page ...
Try reloading the files for:
/includes/modules/checkout_whatever.php
there are a few in there ...
Are you using any kind of SEO/rewrites? If so, turn them off and see if that is the issue ...
If that still doesn't fix things, temporarily switch to the Classic Template and test your checkout ... this will rule out your templates and override files ...
Also, can you post all of your settings on the Discount Coupon for the 100%? You can mask out the discount code itself ...
Linda McGrath
If you have to think ... you haven't been zenned ...
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