Thank you for your patience! Here's the fix.
authnet-sha512-fix.zip (29.4 KB)
You'll see three files in the zip. They go in these folders of s2Member Pro:
Although you should not have any problem from these, it's always best to make a backup before your test.
Please report anything you find not working, or that could be improved. Your feedback is very appreciated and will be used to improve the fix before implementing it in the next release.
Looking forward to your results!