Hi @Lemlurker,
You need to have a closed volume (no open surfaces or edges - we recommend to use solid modeling to make sure that you will have a closed volume), avoid intersections and overlapping of surfaces. That will ensure that the meshing process works. Please provide me with the link of the project and I will see where the mistake is.
In addition to that, cutting the model into half does not make it watertight since the end is now open and not closed. Make sure that everything is closed as mentioned in post 2.
Additional hint: That is also important for the Material Point as @dheiny explains in this post
This also makes clear why it's important to have a closed CAD model ("watertight") when using the snappyHexMesh operation because otherwise, it might simply mesh both "sides" of the model - inside and outside.
Best,
Jousef