mtube: Thank you very much for your quick reply.
In Manage >> Ad Units I did see the old ads, but the HTML code ads were empty and the classic AdSense ad units did not work anymore. I guess this means the import must not have been working correctly, as you suggested.
I had to delete all the old ads and to recode the HTML stuff, which was a bit annoying, but that made it work for me in the end.
The classic AdSense units didn't work and produced the error I quoted above, but I found out that the plugin works with the new AdSense ads. So I removed the classic units and replaced them with the new ones.
FYI, I'm not sure about the specific version of the plug-in I used before, but it was a 2.x version.
Thank you again for your great support.
Have a nice day,
Dom