This is a known problem specific to Mercatox.
There was a thread on Twitter earlier today where one of the RaiBlocks developers confirmed that they are working with Mercatox on some issues they are facing with their node. Should hopefully be resolved sooner rather than later, but not sure on specific ETA.
This is not typical of XRB transactions, but is some kind of issue with Mercatox right now.
See the full reply chain on Twitter –