Hi Michael,
thank you
I noticed the "extra" edge, but I did not zoom it soo close to see the that detail. But still I do not understand where the faces inside the "healthy" solid came from...

But in this case I have a new and issue:
Time to time happens the snapping is somehow not precise. In this case I draw a circle exactly on the middle of the edge, and extruded to the opposite edge. The difference is 0,001mm but still enough to casue fatal problems...
Sometimes operations like mirroring, copy etc produces such small differences... Drawing a lot of symmetrical objects related to a fixed center point ends up in having a small "cloud" of center points in area of 0,01mm.
Is there a setting how precisely should objects snap?

Jozef