Hi,
I also face this problem right now..
I had spend days googling trying to fix it but in vain.
Somewhere said to set files permissions from 0777 to 0755 as this is a major security risk. Done that but still..problem remain.
at my case order completes successfully but client sees an server 500 error instead of a "Thank you for your order" or something else..so he thinks that order was not send. But at back office everything are there.
You can see it for yourselves at www.birbilo.gr, also no e-mails send to customer..nothing