401 Unauthorised on API calls with necessary access

Hey guys, wondering if anyone has come across something like this. Getting a strange bug when making some on page API calls. The calls work just fine on my computer, but one colleague has troubles making the exact same requests.

We can be logged into the same account (full sysadmin privileges etc) yet his request will 401 (typically unauthorised).

He runs many more iMIS instances in a browser session than I do, and appears to be caching the RequestVerificationToken from another instance, as if he opens a private browser, it works fine.

Clearing the cache is a temporary solution, however it continues to happen. We are making the API request using the Request Verification Token that is on the page, so in theory it should always be correct. I understand maybe if the session has timed out, but simply refreshing the page without clearing the cache does not solve this.