Hi Abhishek,
I tried to debug the issue at my end . The above error occurs on Chrome Browser & not the firefox however in the debugger the global mbox call is successfully firing and even the mbox.js is successfully loaded on the page in chrome as well firefox.
For better debugging of mbox call : can you check the mbox trace by adding the url with the below parameter
?mboxTrace=window&authorization="your-authntication-token"
You can grab the authentication token from your marketing cloud by going to [Target -> Setup Tab -> Implementation on Left Rail ]
Regards
Parit Mittal