Hi Jules,
We are unable to reproduce any of these problems on our machines. We have tested RC6 very intensively during hours of continuous work with huge images, complex projects and long processes, without a single issue. There were some stability problems in previous release candidates, and a single reproducible sequence leading to a crash in RC5 with the LinearFit tool. However, all of these problems have been fixed in RC6.
If we are unable to reproduce a problem, there's no way we can fix it. The crash report you're providing indicates an invalid memory access (a null pointer in technical terms). In can be a bug in some support libraries used by the system, or in some support libraries used directly or indirectly by PixInsight. It may be due to some interaction caused by running applications or programs, which we have no control over. As I've said, we have tried very hard to reproduce this problem without success.