We have run into this error a couple of times, Rakhi's example below:

Gr4vy one-time donation in IE failed.

https://payments.wikimedia.org/index.php?title=Special:GravyGateway&appeal=JimmyQuote&country=IE&currency=EUR&payment_method=cc&recurring=0&uselang=en-us&amount=2

Error message: There was an error processing your request.
Error reference: 213605618.4

Christine - from Spectrum internet in Oswego, NY;
212890083.1
(RawData) Create payment response {"type":"error","code":"bad_request","status":400,"message":"Request failed validation","details":[{"location":"body","message":"Invalid checkout session ID. Session does not exist, is incomplete or has expired.","pointer":"\/body\/id","type":"value_error"}]}

Etherpad with more data here:https://etherpad.wikimedia.org/p/gravy_invalid_checkout_session_id