Kontact error since upgrading to Buster

[Gary Dale]

I'm getting periodic messages from Kontact as follows:

The server failed the authenticity check (planet.kde-espana.org).


The certificate has expired


This has been happening since I upgraded from Stretch to Buster a couple 
of days ago. Any ideas on how to get rid of it? I don't even know why an 
apparently Spanish site should be involved with Kontact.

It gives me options for "details", "continue" and "cancel" but clicking 
on any of them simply clears the message temporarily.