Can’t resolve vtm.zive.cz

My Omnia regularly cannot resolve vtm.zive.cz - can someone dig into this and check what’s wrong?
Forwarding is disabled, DNSSEC is enabled.

nslookup: can’t resolve ‘vtm.zive.cz’: Try again

Thanks.

That’s certainly the same problem as for this user DNS resolver dropouts (or hangs?) – all known cases depend on the nameserver from ignum.cz which is somehow serving as a canary for this problem, but what exactly is the root cause is unknown so far. (I’m sorry.)