Without the information to test the script from your site I can tell you why that error most often occurs. It occurs when the mailing program is either not getting some information, or there is a syntax error in the information that is getting passed to it.
You will want to go through all the variables and ensure they are being created and passed correctly.
I hope this answers your question. If you have any more questions or information specific to the issue please leave a comment below so we can further assist you.
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