Even though the API username, pass and signature are correct, and sandbox mode is activated, too!
What could be wrong?
I'm using all the latest joomla 1.5, joomfish and virtuemart versions.
I never had issues with API credentials, so i still think you're using / inserting wrong credentials.
Remember that SANDBOX API credentials are different from real API credentials.
And remember to trim extra spaces before / after the strings.
I decided to use the legacy thingy. --"
Thank you anyway.