RE: Postman API connection fail
Hello,
I think it can have a lot of reasons. First of all, I know it is obvious but do check: In Postman that the request is set up for Basic. Or to inherit from parent and then the parent is set up for Basic. And the User Name is as it shows in the BC client, and then copy in the web service key:

I know that this is wha you say you do already, but sooo many times, problems in Postman is down to tripllechecking details.
Then see if the BC Service Tier is set up to allow "Use NTLM Authentication":

This tick oftem makes soap and odata connections start working.
If still no luck, then install Telerik Fiddler, and run a trace and see if that catches some more details.
Also looking forward to hearing from others if they have seen other reasons for this.