I have the same problem as Sutex. All I get out of the client is invalid credentials as well.
I am using EDMC 2.4.3 on Windows 10.
A bit more diagnostic information would be a help. Does EDMC allow for tracing or provide detailed diagnostic information somewhere ?
Not that I am aware, but I'm sure there is logging for it somewhere either on the client <(in your directory tree for EDMC) or the EDMC website under your account possibly.
I am not an expert on this piece of software or the API.
I also use Windows 7 as opposed to Windows 10, so that may have some influence on how EDMC performs, as well as how the API and logging are implemented.
In the thread I pointed to, there seems to be many more experts there working on possibilities, but everyone here should know that the Frontier API was NOT DESIGNED for use by third party support sites, and those sites, on there own, figured out how to use the API to grovel and present data on THEIR sites. Frontier offers support on the API for Frontier's use, not third party, i.e. EDDI/EDSM/EDMC and others. This leaves the support sites on their own when changes occur on Frontiers side, and all of us are left scrambling for answers and solutions.
My play is to install the third party stuff on my side (client), get it working and leave it alone until there is some change or update by the third party developer. If it stops working, 99% of the time, it is Frontier's changes which prevent it from working, and there is nothing to do but wait it out.
Done all the bits suggested , nothing is working, API is not working for me just have to wait ....
On your issue, I still believe it is a client side problem. Did you clear your "browser cookie" associated with Elite and EDMC as suggested in the "fix" on the post I included?
The "invalid credential" issue has two reasons, one is clearing the cookies in your browser, which contain usernames and passwords associated with the API issue.
Sorry I could not be of further assistance.