Hi Sebastian - it looks like it's got some pieces that are not fully attached together and so that's why it's not a solid.

You can tell if it's a solid or not in Moi before doing any export by selecting it and then looking at the object type indicator in the upper-right area of the properties panel, here:



That would read as an object type of "Solid" if it was a fully closed solid. For your object it instead says "Joined srf" which means it is made up of more than one surface joined to each other but not fully joined everywhere to make a totally closed solid.

To find out which areas are open you need to set up a script on a shortcut key, the script is here: http://kyticka.webzdarma.cz/3d/moi/#SelectNaked

That script will select all "naked" edges - those are edges that belong to only 1 surface and are not joined between 2 surfaces, so those are where openings are at. It can be useful to put this script as a shortcut on the N key so that you can push N to show naked edges at any time.

When doing that with your model here it shows this:



So something is wrong in that area of the model.

One thing I noticed is that this little piece here seems to be an extra surface that is overlapping right over top of another one:



If I zoom in there:



And delete it you can see that the other piece is not just a piece next to it, the other piece actually extended the full length underneath that little other piece:




So it looks like you had an extra piece in there overlapping on top of other things and that can make for confusing joining in regions like that, some edges were connected to the big underneath piece and some were connected to the little piece that was overlapped right on top.

Hope this helps!

- Michael