Just a "me too" reply here: I am having the same problem when trying to call taSopLineIvcInsert with SOPTYPE=4 and DOCID=RTN.
I have no problems creating an order (SOPTYPE=1).
In my case I am directly calling the eConnect stored procedure:
EXEC @RC = TEST.dbo.taSopLineIvcInsert
@I_vSOPTYPE = @I_vSOPTYPE,
@I_vSOPNUMBE = @I_vSOPNUMBE,
@I_vLNITMSEQ = @I_vLNITMSEQ,
@I_vCUSTNMBR = @I_vCUSTNMBR,
@I_vDOCDATE = @I_vDOCDATE,
@I_vITEMNMBR = @I_vITEMNMBR,
@I_vUNITPRCE = @I_vUNITPRCE,
@I_vXTNDPRCE = @I_vXTNDPRCE,
@I_vQUANTITY = @I_vQUANTITY,
@I_vUpdateIfExists = @I_vUpdateIfExists,
@O_iErrorState = @O_iErrorState OUTPUT,
@oErrString = @oErrString OUTPUT
SOPTYPE = 4, and all other values seem to be correct. (These same values work when SOPTYPE = 1.)