"API_ERROR: Unknown error" when using `npx netlify-cms-proxy-server`

**Describe the bug**

When using netlify local backend, and navigating to edit pages, it fails with `API_ERROR: Unknown error". (On just viewing the edit page, not on save.)

**To Reproduce**

1. `npx netlify-cms-proxy-server`
2. `gatsby develop`
3. Go to `/admin`.
4. Click on a page to edit, in my case "Teachings", click on a teaching.

**Expected behavior**

Expect to be able to edit pages.

**Screenshots**

**Applicable Versions:**

- Netlify CMS version: [e.g. 2.0.4] netlify-cms-app@2.15.17
- Git provider: [e.g. GitHub, BitBucket] GitHub
- OS: [e.g. Windows 7] MacOS (M1)
- Browser version [e.g. chrome 22, safari 11] Chrome 91.0.4472.77, Firefox 89.0

- Node.JS version: 14.3.0
- NPM: 7.10.0
- Gatsby: 3.6.2

**CMS configuration**

**Additional context**

```
API_ERROR: Unknown error
t http://localhost:8000/admin/netlify-cms-app.js:54
i http://localhost:8000/admin/netlify-cms-app.js:54
request http://localhost:8000/admin/netlify-cms-app.js:493
getMedia http://localhost:8000/admin/netlify-cms-app.js:493
getMedia http://localhost:8000/admin/netlify-cms-app.js:28
f http://localhost:8000/admin/netlify-cms-app.js:34
loadMedia http://localhost:8000/admin/netlify-cms-app.js:34
r http://localhost:8000/admin/netlify-cms-app.js:164
r http://localhost:8000/admin/netlify-cms-app.js:22
componentDidMount http://localhost:8000/admin/netlify-cms-app.js:216
React 12
notify http://localhost:8000/admin/netlify-cms-app.js:22
notifyNestedSubs http://localhost:8000/admin/netlify-cms-app.js:22
handleChangeWrapper http://localhost:8000/admin/netlify-cms-app.js:22
v http://localhost:8000/admin/netlify-cms-app.js:43
waitUntilAction http://localhost:8000/admin/netlify-cms-app.js:58
r http://localhost:8000/admin/netlify-cms-app.js:164
dispatch http://localhost:8000/admin/netlify-cms-app.js:43
authenticateUser http://localhost:8000/admin/netlify-cms-app.js:43
promise callback*t.authenticateUser/< http://localhost:8000/admin/netlify-cms-app.js:43
r http://localhost:8000/admin/netlify-cms-app.js:164
_ http://localhost:8000/admin/netlify-cms-app.js:148
loadConfig http://localhost:8000/admin/netlify-cms-app.js:43
r http://localhost:8000/admin/netlify-cms-app.js:164
_ http://localhost:8000/admin/netlify-cms-app.js:148
js http://localhost:8000/admin/cms.js:14361
factory http://localhost:8000/admin/cms.js:48787
__webpack_require__ http://localhost:8000/admin/cms.js:48180
http://localhost:8000/admin/cms.js:49495
http://localhost:8000/admin/cms.js:49499
netlify-cms-app.js:34:39826
f http://localhost:8000/admin/netlify-cms-app.js:34
(Async: promise callback)
f http://localhost:8000/admin/netlify-cms-app.js:34
loadMedia http://localhost:8000/admin/netlify-cms-app.js:34
r http://localhost:8000/admin/netlify-cms-app.js:164
r http://localhost:8000/admin/netlify-cms-app.js:22
componentDidMount http://localhost:8000/admin/netlify-cms-app.js:216
React 12
notify http://localhost:8000/admin/netlify-cms-app.js:22
notifyNestedSubs http://localhost:8000/admin/netlify-cms-app.js:22
handleChangeWrapper http://localhost:8000/admin/netlify-cms-app.js:22
v http://localhost:8000/admin/netlify-cms-app.js:43
waitUntilAction http://localhost:8000/admin/netlify-cms-app.js:58
r http://localhost:8000/admin/netlify-cms-app.js:164
dispatch http://localhost:8000/admin/netlify-cms-app.js:43
authenticateUser http://localhost:8000/admin/netlify-cms-app.js:43
(Async: promise callback)
authenticateUser http://localhost:8000/admin/netlify-cms-app.js:43
r http://localhost:8000/admin/netlify-cms-app.js:164
_ http://localhost:8000/admin/netlify-cms-app.js:148
loadConfig http://localhost:8000/admin/netlify-cms-app.js:43
r http://localhost:8000/admin/netlify-cms-app.js:164
_ http://localhost:8000/admin/netlify-cms-app.js:148
js http://localhost:8000/admin/cms.js:14361
factory http://localhost:8000/admin/cms.js:48787
__webpack_require__ http://localhost:8000/admin/cms.js:48180
http://localhost:8000/admin/cms.js:49495
http://localhost:8000/admin/cms.js:49499

```

`POST http://localhost:8081/api/v1`
request: `{"branch":"master","action":"getMedia","params":{"branch":"master","mediaFolder":"static/media"}}`
response: `{"error":"Unknown error"}`