The API-code doesn't work for Envato Toolkit

I have a couple of sites where I successfully been able to use the Envato Toolkit. I’m now trying to use it on a new site but when I save the settings with my username and a new API-code it says Unknown user. It’s the same username as on the other sites with different API-codes for each one. I did it on another site yesterday and then it worked smoothly.

What can be wrong?