Hello Joachim
Many thanks for your advice.
We did have another ads.txt plug-in active. Whilst this wasn’t an issue before the update we have disabled and deleted this second plug-in and we no longer have an error message within Advanced Ads : )
Google AdSense still contains an error message at the moment, but we guess that may take some time to correct itself.
Thanks again, your assistance is much appreciated!
TLCB