That’s interesting, afaik no-one else faced this issue.
So to clarify, you can save PNGs on all other packages, but not on the Season #3 Female Rider?
Did you try different item combinations, saving only the default character that loads when you open the software?
Can you try saving with a transparent background (the one with the crossed zero background)?
Can you Save the character instead of an Export, and Load that back later? If you can, are you Export the loaded character without issues?
If you have the PSD Exporter, can you try saving it as a PSD instead?