Api Fails Authentication?

I have used the cs-cart API on several sites in the past without any issues, but I have one new site that I can't seem to get it to work on. I keep getting
Message from CS-Cart API: Unauthorized
I have tried several Admin email addresses and their associated API key's, and all fail authentication.
Any suggestions, can this be a server issue? Please help.
CS-Cart 4.3.9

I upgraded to CS-Cart 4.3.10 and we are still having the same problem.

Any suggestions?

Can only suggest you look at error_log and your Apache access_logs for an indicator…