Unable to retrieve 4.x/5.x key from v3 key

Hi. I purchased FlashFXP a long time ago and just reacquainted myself with this great product. I have a v3.x key and after installing v5.4.0 Build 3970, I pasted my v3 key into the software and clicked 'get key' The screenshot attached shows the result.

I'm sure things have changed quite a bit since my purchase in 2004, but I would like to use this product to it's fullest.

Try https://www.flashfxp.com/portal/key