Anyone else suddenly having resolution errors?
My host all looks fine in the portal, but none of my systems, and tests with external tools can resolve my x.myddns.rocks domain!

Same here. The myddns.rocks DNS zone doesn't seem to have any NS records anymore. The whois information seems correct, so it seems like a technical issue of some kind. I manage some .rocks domains of my own, which are still working, so it doesn't look like a global issue with that TLD.

I have exactly the same problem.
Another thread about this: https://www.dynu.com/en-US/Forum/ViewTopic/myddnsrocks-not-available/10804

Working now.