Knot resolver not resolving (NXDOMAIN)

After a while I’m stuck again in situation when queries to some domain are not resolved and lookup finishes with NXDOMAIN. Previously I tried to enable HTTP tracing module and now I managed to retrieve kresd verbose log. The domain in question is Request ID in log is 65417. The log is attached below. Can anybody decipher what’s going on? I’m a bit lost in the log.

Log file:

One of their servers didn’t reply at all, several times in a row. kresd clearly thought there’s noone else available to ask, which was probably a mistake. (The log does not show enough information to be sure whether there was another option. I checked 2-3 hours after you captured the log and there were two IPv4 addresses serving the zone, but the one not replying was not present, so clearly there has been some change in their DNS in the meantime.)

We used to have these problems in older kresd versions; I thought they were practical gone nowadays. In any case, the nameserver selection code is being completely redesigned now, tentatively planned to be finished before July.

Thank you for the analysis. The issue is usually transient, this time it disappeared before I finished the initial post. But it happens occasionally (with various domains) and it’s annoying. So there’s nothing I can tweak now, right?

There is one related setting, but I don’t think it can help in this case.