Hello Steve, I suspect this lookup error is because you have knewstuff packages in version 5.54.0-1 installed, which is not yet in testing (and barely in unstable).
Could reproduce it with packages below installed: http://ftp.de.debian.org/debian/pool/main/k/knewstuff/libkf5newstuff5_5.54.0-1_amd64.deb http://ftp.de.debian.org/debian/pool/main/k/knewstuff/libkf5newstuff-data_5.54.0-1_all.deb Could you test if downgrading knewstuff to testing version 5.51.0-1 solves your issue? Kind regards, Bernhard