A hair on the Blue Filter !!!!

When we took flat images, we could see a weird artifact on one side:

It looks like something not quite on the lens or the filter - but slightly in front or so.

And even worse!!! It doesn't seem to be stable. Here is a different flat:

And of course these also show up on the actual images. I tried various things but couldn't find a way to eliminate them :-( So for now, we have to live with them - which means cutting out the left hand side of the images...