The failure to decode json usually indicates that your payment gateways WAF is blocking the request due to the what is being sent in the payload and it's usually related to what the competition is called.
Ultimately things like "Echo Dot" etc will be blocked as it contains the world "Echo" which a command for printing to the screen. Equally "Curlers", the word "Curl" is a command is also special for web request so it is blocked.
Things that should be avoided include but not limited to:
echo
exec
system
select / insert / update / drop (SQL keywords)
<script> / javascript:
alert()
onerror= / onload=
cmd / sh / bash
||, &&, ; (command chaining)
If you remove the word from your competition title and URL, your customers should no longer get this warning.