HI Andrew,
Thanks for the feedback. In the mean time i did some tests and have some results i did not aspect.
On your advise i switched the language in Joomla 2.5.14 from Dutch to English. I have noticed that the same error appear while logging in the back end. After clicking on some components, modules or articles the message never show anymore.
During the test i have not switched the language anymore and went to the installed version 2.0.22C virtuemart where i have created 3 products before. After the swithch from Dutch to English i have a SQL error and the products are not visible anymore. when i switch back to Dutch the installation is back to normal. and 3 products are available.
When i try the free version before i had not such error as plgauthenticationplg_authentification_simplemember. I am working on the moment on a pilot in a subfolder. also i have used for installation for Joomla the Installatron application on the server.
Hope this information helps to finish my website for a go-live.
Already thanks,
Mario