Someone at the photo store too, figured that the weird effect was caused by some kind of interference between the layers of glass.
It became clear to me, after shooting some test shots outside the photo store with a tryout lens provided for the occasion, a Hoya, that a much more expensive filter of, obviously, a high quality, would not present the disturbing bokeh, however, at this moment I need to cough up money for other essentialities. Then I also tend to agree with the non-filter-is-best crowd. So I simply bought a lens hood for a low price. It provides protection and the image quality is great.