Hi Stefan - the booleans are more oriented around working with solids where the different pieces need to be intersected with each other and have pieces cut up and some pieces removed.

In your case you don't have anything like that which needs to be cut with each other, so it would be better for you to use Edit > Join to glue the surfaces together, instead of using Boolean union. Join does not try to do any of the extra intersection and cutting work that will happen with the booleans.

Also one of your surfaces down in the bottom area is a super skinny little slivery piece, that's probably the one that is confusing the booleans.

I also found that one small area of your new blue piece along the bottom did not seem to come close enough to be joined properly.

So here I've attached an edited version of your object in your last post which is ready to be joined. To finish it up, select all the 14 surfaces and then run Edit > Join. Now it's almost finished except for the hole in the bottom - that's where your new surface did not quite come close enough to be joined so it is easier to rebuild the planar bottom piece rather than trying to rebuild the blue connecting piece.

The quick way to close off a planar opening like this is to select the whole object and then run Construct > Planar, that will put in a planar cap in that bottom spot and then you'll have a finished solid.

So basically just select the surfaces, run Edit > Join, then run Construct > Planar.

Hope this helps!

- Michael