This error appears when you try to submit an initial recurring transaction to PayPal’s Payflow Pro gateway.
I was puzzling over this issue for days and had no clue why it was happening. The form worked fine most of the time, and once in a while I would get this error. A similar setup for one of my other clients worked fine 100% of the time. So after some googling and code-digging I found the root of the problem: the “ProfileName” field was being dynamically generated, and if it had strange characters, like “&” (ampersand) or was too long, that would trigger the Start/Nextpaymentdate error (because the “Start” field immediately follows the “ProfileName” field in the string sent to PayPal). To fix this, make sure your “ProfileName” value is something static, like “MonthlyPayment”. That should take care of this.