I did the update to the current version now. But it occurs that websvn folders still are not garbage collected. Until now there are only two such folders remaining. Before updating there were much more. Such a folder contains one folder from the SVN followd by a revision tag in the name. Besides that two archives are contained in each websvn* folder. I can't narrow down when those folders are created because the current websvn* folders contain folders from my SVN which I haven't clicked on in the websvn interface.
I will oberserve the behaviour.