I am using MyFilms in Avalance skin, as not very strongly implied in my original post. But I do have to report some progress.
I unistalled the DVDart plugin, then did a search on my C: drive for *DVDArt*.*. That returned a number of items that are leftover to the installation, and particularly some xml and db3 files that I deleted.
Then I did a fresh installation of the plugin, and when I started the config from the desktop I didn't get the error message. Then I made the configuration and this time left all the original art location folders as is. Started the importer and got the same result. A populated importer view with some items as movies in a weird name (tt12343434- looks like IMDB ID but was captured as movie name, plus some weird files names as movies. Got to note that all series where listed properly). Thought I hit a dead end since I could still see no items under the movie series or music tabs either with or with missing artwork. But then I closed the config and restarted it and there was the miracle. The previously acquired list under the importer tab was still there, and this time a downloading of data started. To cut the long story short now my Series (TVSeries plugin) properly represent clear art (except DVD art that I think is not in the options for series anyway) but there is no sign of Clear art. clear logo or dvd art in the movies although most of them I believe are recognized and have DVDart, clear art and clear logo visible in the configuration.
So one step closer to get this to work flawlessly. Series resolved, Movies still at 0. Any suggestions? Do you require any logs?