Hi @segidas thanks for posting on the Forum and welcome to the SimScale Community 
Could you please share your project’s URL so we can take a closer look? Here’s a guide on how to do that: Guide to Clear and Understandable Forum Posting
As a general note, however, we usually ask users to avoid using STL files for simulation using the Incompressible solver, as the faceted geometry often contains self-intersections or the facet-splitting isn’t 100% successful and a few of the facets are still mantained.
Could you try exporting your geometry as STEP or Parasolid instead?
Cheers
Igor