Thank Roxanne for your very quick reply. I will try to describe the way I proceeded:
1 First I reinstalled via BWS without the Eilistraee mod you suggested. Sadly I ran into the same error as before, suggesting that Eilistrae is at least not the only problem (if at all).
2.1 According to the earlier solution you described, I manually uninstalled SandraNPC and reinstalled component 1-5, with the option to ask for every component. The Manual installation ran perfectly to this point.
2.2. Changing back to BWS I quit the manual installation and told BWS to continue. BWS provided the next error regarding a problem that some components have to be installed all at once. This time BWS only provided the option to skip the component.
3.1. I switched back to manual installation again, uninstalled SandraNPC and could again successfully install the package (this time the whole component, without asking for single components).
3.2. Back to BWS, I told BWS to skip
4. It seems that BWS registered the manual installation an continued with the next packages.
I was wondering regarding the issue in 2.2. In the beginning I told BWS not to install the different components in batches, because that lead to memory issues during the installation of SCS. But SandraNPC seems to be designed to be installed in one go, as far as I understood the error message? In this case could it be that by deselecting the batch installation of BWS I messed up the install procedure of SandraNPC, thereby producing my initial bug?
Or is SandraNPC designed to work, if I skip some/many of the subcomponents of this mod?