Hi TIG,
thanks for your reply. Here are my answers to your questions/suggestions:
I have full rights (I work as administrator account) to all relevant folders.
It does not matter which plugin (beside the ones delivered with the installation) I try... e.g. soap skin and bubble, joint push pull... always the same effect. I can start SU exactly one time after adding any plugin. By the second startup and each try later on I get that bug splat screen, that tells me that there was an error (but with no information what kind of error). It happens right after the user interface has been drawn.
I have a couple of times done a full uninstall and reinstall while trying to find a solution. It is the only way to get free of the splash screen after adding a plugin. Removing a plugin or replacing the plugin folder with the original one (that one containing only the plugins that come with the new installation of SU) does not help. Strange.....