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Suddenly I'm starting to see this message on pages where I previously never had a problem. Does it mean that Pi-Hole isn't resolving DNS properly?

Are you seeing something blocked in the query log?

Nope, it's showing as resolving (in green) for two entries.

Then Pi-hole is correctly resolving the domain.

Check if other domains used in the page are being blocked. If Pi-hole is not blocking domains from this page, the issue is elsewhere.

Virgin Media's V5 router doesn't play nice if you use unbound. I had to put mine into modem mode and use another router. Numerous domains didn't resolve. Gov.uk was the obvious one failing. If you are not using unbound try a different upstream DNS in pihole.