Hi Andras - could you please explain a little more about what exact offset you're trying to do?

I loaded your file and tried offsetting all the individual surfaces by 1 unit and they all seemed to generate an offset.

Then I tried joining them together into a solid (using Edit > Join command) and that also seemed to offset.

One thing that may be causing a problem is that there seems to be an extra degenerate surface in your model - I noticed when I did an area select that the selection indicator said "7 Surfaces" were selected, but there are only 6 big surfaces that seem to be present. If you select the 6 main surfaces by clicking on them:



And then delete those, you'll see the 7th surface is this little fragment that looks like a line here:



So probably that little degenerate surface (it may be a surface that has all of its area trimmed away from it so it's more like a line instead of a surface) is causing some problems?

- Michael