The fact that the files are there and black and the right size is a real clue.. it seems to rule out a problem with actually saving the file, and more to do with the png data being somehow problematic.. I wonder if it could be something to do with the auxilliary data in the png file (exif, file comments) somehow causing it problems..
Before I work on that hypothesis, can you try redownloading the latest alpha version (see link in previous post above), and then assuming the error is still happening can you tell me:
1. Your operating system.. win10 or winxp?
2. Does the error happen with every capture, or are some successful? Any pattern as to which work and which give error?
3. What are your settings on the File Saving tab? Does changing the compression level make the error go away?
4. If you change format to jpg on the File Saving tab, does the error go away?
Regarding settings -- in the installed version the settings will be in your MyDocuments\DonationCoder\AutomaticScreenshotter directory, and will not be deleted when you uninstall.