Is it just a dispute that the buyer has opened or has he escalated it to a claim?

If its a dispute then it can only sit like that for 20 days then it has to be resolved, I.e. escalated to a claim, closed etc. my understanding is if you have proof of delivery then you can escalate it yourself to be finalised. If you don't then it makes it a bit harder and you might have to sit out the 20 days and see what the buyer does.