I planted one European Elderberry bush in the hope to harvest many berries in the fall. The bush was full of white blossom clusters but only very few fruits developed. What could be the reason for this? I suspect that there were no bees or other pollinating insects. How could I attract bees or insects to the bush short of setting up a bee hive?

I have the American Elder and have no problem with it setting fruit. However, the birds like the fruit and strip the bushes clean quite fast. Are you sure there was poor fruit-set or did the birds get to the berries before you did?!

There were almost no berries on the bush. On one cluster there were about 20 berries all the other blossom clusters (about 50) were not pollinated.

You said 'the bush' - do you only have one? That's probably your problem - to maximize fruit set, it's best to have more than one for cross pollinization. I have 6-8 plants (I'm trying to hide my ugly garden shed!) and I have lots of fruit. Mine are Sambucus canadensis, not nigra. I think the two species will pollinate each other but you might be best to get another Sambucus nigra and see what happens.