After a long time I again had the same error as described earlier:
I had a problem with one scene where the rendering process stops during final gather calculation. The "Computing final gather points"-bar shows 99,9% and the rendering process stops for several hours (but processor worked between 10 and 15%, not idle), but then it goes on with rendering and ends. Or stopped completely sometimes.
One answer of the forum was, that it could be a Vray issue, but I render with MentalRay (next time I should be more precise).
Now I read the Manual of Forest Pack once more and found the hint, to enable "Use placeholder objects" (Render setup-->Processing-->Translater Options). It seems, that this was my problem. In one scene accidentally this option was not marked. After that I saw, that the memory consumption was only 5,6 GB vs. 15,3 GB before that. May be that my 16GB render machine stopped because of that, but the system panel showed now swapping. And during the render process the processor load is constant 100% all the time (in my investigations before that with a very small render region the processor operated between 10-15%).
Didn´t know how important this "small" adjustment "Use placeholder objects" can be.