Hmm actually that callContextHeader you are passing isn’t a valid header. Not sure why it now “works” for you if I had to venture a guess is you logged in into Epicor and changed to that company so your session got updated.
The actual Header you need to send is CallSettings as documented here
There is a lot of stuff documented, here on the forum and on Epicor Help and on the various Guides.
IF you haven’t done so I recommend a thorough browsing of the “Experts Corner” in this forum as well as looking at the documentation SWAGGER, the Epicor Help Regarding REST, EpicWeb Documentation such as the Customization Guide and Exprience User Guide as well as the Web Services Technical Reference.