Thanks @optiplex, it worked for me aswell!Found this on the ED Market Connector site and it worked for me
@jamsoft I have just had the same issue. I solved by going to the [Frontier auth service website](https://auth.frontierstore.net/), logging in, deauthorizing EDMC and then trying to get credentials again through EDMC. Frontier asked me to authorize EDMC again and then it worked.
hope it helps you to