When trying to process a credit card through the Payflow gateway, receive the following  --  "Generic processor error: 11604-Request for the specified Message Submission ID (specified in MSGSUBID parameter) cannot be started until the previous request finishes."  What would cause this error?

Here is what we send and receive.

Request

TRXTYPE[1]=A&ACCT[16]=411111XXXXXX1111&EXPDATE[4]=1224&CVV2[3]=XXX&TENDER[1]=C&INVNUM[16]=INV-11787-Z5Z5K2&AMT[4]=5.00&CURRENCY[3]=USD&COMMENT1[6]=001880&COMMENT2[16]=UX Authorization&BILLTOSTREET[17]=290 Bremner Blvd2&BILLTOCITY[7]=Toronto&BILLTOSTATE[2]=ON&BILLTOCOUNTRY[18]=Korea, Repu[Removed. Phone #s not permitted]sy&BILLTOLASTNAME[9]=Schroeder&USER[9]=161474752&VENDOR[9]=161474752&PARTNER[6]=PayPal&PWD[11]=XXXXXXXXXXX

Response

RESULT=1000&PNREF=B70PAF34381B&RESPMSG=Generic processor error: 11604-Request for the specified Message Submission ID (specified in MSGSUBID parameter) cannot be started until the previous request finishes.