StarTools

I have noticed that if you register an image in PixInsight and then resave that image as a Fits file (bit depth doesn't matter) it will cause a lock up when you try and open the Fits file in StarTools. It appears to have something to do with the added black area. If you save it as a 16 bit Tiff then startools will open it just fine.

Also if you use the HaRVG-AIP script the resultant file if saved as a Fits will lock up StarTools and if saved as a Tiff can't be edited in StarTools (appears as a deep blue image no matter what you do with a few white stars).

While it isn't pretty (I know), you'll find that the file will eventually load.
What's it's doing in the background is filling missing pixel values with 'plausible' data in the context of the rest of the image, so that it can be successfully processed; the black data isn't actual data at all but is encoded as a 'missing values' or 'Not a Number'. In a way that's actually much more helpful than just generating a 0 value pixel, as it gives lets StarTools a way to mitigate the adverse effects of leaving in 'missing' data and take counter measures.

If it were not to do that (e.g. take counter measures), some algorithms in StarTools may start generating ringing artefacts or it may cause similar undesirable consequences, exactly like leaving stacking artefacts in would.

The best course of action is to make sure PI, DSS or similar does not generate these areas in the first place (there are settings that govern this behavior) - you will invariably end up cropping the image anyway!