REST custom - 400 bad response error

Hi,

I'm building an API monitoring sensor for the first time and getting the error "Cannot load endpoint

https://**REDACTED**/MGRESTService.svc/json/idoinfo/BGTaskHistories?readonly=true&rowcap=1: expected status 200 OK but got 400 Bad Request."

I'm using postman to manually query the same API, so am confident the URL is correct. The sensor doesn't automatically get a token, so I'm manually pasting this into settings at the moment. I've tried passing the token as a custom HTTP header also. If I take the URL (which I've deliberately redacted here) from the error and paste it back into postman it functions correctly. I see the colon appended to the URL, which I assume is PRTG formatting the error - or is this relevant?

The sensor is configured with the 'channeldiscovery' REST template.

Any help appreciated. Thanks.

api json rest