You Brits have to learn that national boundaries determine who may sell the tickets-not what company is written on the side of wagons. After all an Austrian stamp will also not be accepted when sending a postcard from Hrvatska to Österreich (thats Croatia to Austria). Yes, there are some exceptions to that now, but in East Europe often old habits die hard. A neutral comp. like loco2 etc. can go around this-but only if the state-entity allows it to. State rail companies respect this mostly still. After all trains are not like planes-certainly not for ticketing+fare issues.
The overnight trains to/fro Croatia (also those from Múnchen/DE) are very, very often complained about as not being bookable online.