This has definitely become more common. It seems to arise in two quite different situations:

as a specific and immediate response to some hosts when a download connection is attempted (often, without going through the "connection" routine). It is more usually the first response but sometimes it comes when attempting subsequent downloads from a host following a browsing (in this latter case, I speculate that it is a reaction to exceeding the host's download limit since it cuts off all current downloads from that host). Once I get this response, no amount of fiddling will change the reaction — the source is forever "bad"; and

if I am cycling too quicky between kill and continue for stalled downloads — I speculate that the command to recall the incomplete file for continued downloading fails to reopen the incomplete file in time to accept new data (at least this problem can usually be cleared by refreshing the search).