Answer
As things currently stand, the browser's own developer tools (Network tab, inspecting the failed request/response) are the only reliable way to trace down which specific permission is missing — IFS's own R&D feedback on this is that a built-in 'here's exactly what's missing and how to grant it' diagnostic is deliberately not exposed at a normal permission level, since that level of detail could be misused; it's treated as something closer to a debug/diagnostic-level capability rather than a standard admin one. Keep this in mind when scoping how much time to budget for troubleshooting complex permission-set structures — there's no shortcut beyond the developer-tools approach today, though this is an area IFS has said it may revisit in a future release.