Hi krass, and yes like OSTexo mentions above, the problem is that there are some teeny tiny little slivery surfaces in that area, if you zoom in very closely you can see that there's a kind of little tiny slot or shelf in the side of your shape. It probably came from doing booleans with some pieces which were not quite aligned with one another:






There's a little tiny "roof" like piece at the top of that slot, and it does not run the whole length so it may have been 2 pieces that were at some slight angle to one another when they were booleaned together or something like that.

It should probably be possible to repair that area of the model by doing some "utrim" and retrim type techniques, I will try to help do that for you a bit later today. If you want to try it yourself there is a tutorial here that describes the general techniques to use for object repair:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael

Hi krass, I've attached a 3DM file with a repaired version of your object, it is now a solid.

To repair it I removed that little tiny kind of slot opening that I showed in the screenshot above.

The first step for removing the slot was to zoom in and delete the little tiny roof piece of it. To zoom in to see that you have to use the "Zoom area" tool and get centered in on the top of the slot, and then zoom in a fair ways and then it's possible to select the little silvery top piece of the slot and delete it:






Then the slot was cut out of this side part here - select this face and the one on the opposite side and delete those too:



When deleted your object will look like this with 2 big holes on either side:




Now to finish the repair those holes need to be filled in with a new surface. In this particular case, those holes are planar areas of the model and those happen to be very easy to fill in, there is a tool Construct > Planar that will fill in planar openings like that by making a trimmed plane there and joining it into place. So select the object and run Construct > Planar and then those holes will be sealed off and you'll have a repaired solid object.

Those are the steps that I did to make the attached repaired 3DM file. Hope this helps!

- Michael

You guys are so cool!
Thank you very much for your advice and help, I appreciate your efforts.
Michael - you're the best! ;))