Hi Pavel and Martin,
Sorry, that you have encountered this issue. Problem is that IFC plugin thinks that boolean cuts are made wrongly.
i.e. the boolean engine that is inside IFC plugin, fails on those cuts. Result could be like those in images, extended wall.
We are working on the IFC plugin on this, but results are not expected any time soon.
For you it would be best to try to fix the cuts in the model. Most common reason for them to fail is that the cut part's surface is on the same plane as the wall's surface and that can cause those surfaces to actually be a little bit off and that would mean that the cut is not perfect.
Sorry that I cannot help you more.
Best regards,
Marko Myllymaa