So finally we figured out what the issue was! 
For a couple of weeks ago, we installed a CORS module on the IIS and changed the web.config accordingly. However, the changes in the web.config related to the CORS module did not work at all. So we removed the CORS module part again. But what we realized was that the IIS server needs to be rebooted after a module is installed. So after a complete reboot, we could add the CORS module section to the web.config which resulted in a working API 
ashokkarale
Anil_G
Yoichi
Kismet_Tosun
aravindbalineni123
lrtetala
postwick
Manisha24
Melroy_Dsa
SorenB