I have made a REST API user with the permissions for reading volumes and svms (via security login rest-role on an ONTAP 9.6 system). The commands are working perfect but this user can't read /docs/api. Is there an additional permission for this?
After adding that, the docs page at /docs/api will load. However, there is a bug that I found while reproducing your issue. After you enter the credentials the first time and the docs load, another credential popup is shown. Entering the credentials again will not work. The workaround is to cancel the second authentication dialog. Then you can browse the docs normally.