Hi Nik.
Too weird if all configurations are correct.
In my work area, many incorrect acvitivies were also because customer modification like user-exits in SD module and I inspected badi badi_sd_cm or other important points in debugging mode.
No other clues are not seen, I suggest to check some program points in debugging mode.
Function SD_ORDER_CREDIT_CHECK
Function MCV_UPDATE_CM_ORDER
BAdI BADI_SD_CM
Sorry for no helpful answer.