Crash after rendering

Each time I render a picture, the program crashes and Rhino ends at the end of the rendering process.
Rhino 6SR14 07.05.19 nXt 5.5.18353.6521

OK - I tried the same file in Rhino 5, same crash effect - strange. I checked the material library and deleted some unused, still the same effect. Besides - command line window an toolbar disapeared every crash-time.
Now suddenly - everything seems to work as it should do. I don’t know why…