I am getting this error, when paypal tries to post back to user manager a payment. I have searched and did’ent find anything. If I am having this issue I am sure someone else will. Ensured that the public host is setup right, SSL is on, tried with and with Secured Response.
You REALLY sure your DNS settings are correct and reachable? I had that once, turned out our upstream provider had switched one of the two DNS servers to a new IP and we didn’t know - not until we got this problem, so every now and again DNS lookups were unreliable. But that was with a v3 UM. What is this? UM v4.6? (Odd though that the old DNS IP was still working, in the sense we could perform DNS lookups on it manually - go figure!)
In this case, the DNS was resolving, at least, I am using it to do setup on auth.net. Auth.net works fine, just not paypal.. Maybe paypal onl updates there DNS every so often?
switch to non secure response, run packet sniff. Then look at the interaction between UM and PayPal? Can be very illuminating! 