PayPal Errors

I know there's a few topics about this, but none of them had an answer that helped me...

I can't get PayPal to work either in Sandbox or Live mode. I have my API info for both live and sandbox accounts and both are having the same results:
- I get sent to the PayPal payment page as per normal (which is clearly working to some extent because the address, logo & price are all there)
- I can login to pay as normal
- when I get redirected back to the site it has an error message that just says "Whoops, looks like you skipped a step! Please go back and try again"

I did get that 10002 error when I tried to use my live API in sandbox, but since fixing that I haven't received any API specific errors.

Please help!
I'm going to turn PayPal off for now because the site is live and people are trying to purchase stuff, but if you want to test it directly please let me know and I can turn it back on for a time.

http://www.dreampowercostumes.com