When doing long exposures, you usually stop down to a small aperture to be able to lengthen the exposure. If you were at f/16 or f/22 you can get blur due to the diffraction limit of your lens.

If you look at twigs in the background, you can see they look almost double. Unless there is some sort of internal reflection due to the filter, it looks like camera shake. Sounds like you did everything right (remote release, mirror up etc). Are you sure your tripod was well set up and locked down? How long was the long exposure?

So to troubleshoot this, I would do another experiment but keep the lens at f/5.6 or f/8 to reduce softness due to diffraction. Also set up the tripod securely and try a range of shutter speeds from long (10+ seconds to eliminate anything that might be caused by releasing the shutter) to short (1/30 or 1/60 if you can, to reduce the opportunity for wind or tripod movement.

You can put a filthy UV filter with fingerprints all over it on a lens and not see much difference in image quality, I would doubt a filter would directly cause blurriness. But I know from experience that diffraction can cause a lot of softness, and in your case it really does look like camera movement.