PayPal Checkout Express Module v3.1 issues

[grantlouie]

I'm having random problems with some customers checking out from our online store using the PayPal Checkout Express module.

They either get error messages from paypal that the order has been paid when in fact it has not or when they attempt to submit a payment via paypal it kicks them to our online store and empties the cart (but no payment has been processed?).  These errors keep on happening on the same customer repeatedly?  One fix is I always have to ask the customer to empty their web browser cache/history and to try again, after this it works for them?  Is there a permanent fix for this?   Also, when an error happens,  I get these debug email message below, I don't know if all 3 of these issues are related?:

 

My oscommerce store is v2.3.3.1  and the paypal module is v3.1 API version. 112

 

RESPONSE:

 

Array

(

    [TIMESTAMP] => 2015-08-26T18:43:43Z

    [CORRELATIONID] => c892dd4336be0

    [ACK] => Failure

    [VERSION] => 112

    [bUILD] => 000000

    [L_ERRORCODE0] => 10471

    [L_ERRORCODE1] => 10472

    [L_SHORTMESSAGE0] => Transaction refused because of an invalid argument. See additional error messages for details.

    [L_SHORTMESSAGE1] => Transaction refused because of an invalid argument. See additional error messages for details.

    [L_LONGMESSAGE0] => ReturnURL is invalid.

    [L_LONGMESSAGE1] => CancelURL is invalid.

    [L_SEVERITYCODE0] => Error

    [L_SEVERITYCODE1] => Error

)

 

 

$HTTP_GET_VARS:

 

Array

(

    [set] => payment

    [module] => paypal_express

    [action] => install

    [subaction] => conntest

)