Not really sure if this is some memory leak while saving or something totally unrelated with the program (filesystem corruption?); the file seems composed by an ini file (very first lines) , a pptx archive (there is stuff like themeVariants/variant6/theme/presentation.xml) and part of the original file on the end.
Also, since it happened with an old stable version (0.99.6839), it might be something already fixed.
Original thread: https://en.smath.com/forum/yaf_postst17631findunread_Error-opening-file.aspx