Simple international round trip. hmmmmm
I used to get this when trying MRs for no overnight stay .. MSP-DFW-LAX-DFW-MSP where I got to LAX at 8pm and left at like 11:30pm the same night. LAX seemed to be the issue .. I could do it with other cities but not LAX.
If this is a 'normal' trip with a few days of in-destination-city time, sounds like a call to Web Services.