Subsequent "InternalServerError: Response code 503 (Service Unavailable)" errors

Developer help
1

Running the latest Docker image. Importing a series of posts through the admin API generates the following logging messages (with HTTP 503 errors) although the posts are being created (HTTP 201 response):

[2020-09-26 08:41:53] ERROR Response code 503 (Service Unavailable)

Response code 503 (Service Unavailable)

"The xmlrpc service was unable to send a ping request, your site will continue to function."
"If you get this error repeatedly, please seek help on https://ghost.org/docs/."

Error ID:
    207883a0-ffd4-11ea-8564-775b063361c8

----------------------------------------

InternalServerError: Response code 503 (Service Unavailable)
    at new GhostError (/var/lib/ghost/versions/3.34.1/node_modules/@tryghost/errors/lib/errors.js:10:26)
    at /var/lib/ghost/versions/3.34.1/core/server/services/xmlrpc.js:85:31
    at tryCatcher (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/promise.js:547:31)
    at Promise._settlePromise (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/promise.js:604:18)
    at Promise._settlePromise0 (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/promise.js:649:10)
    at Promise._settlePromises (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/promise.js:725:18)
    at _drainQueueStep (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/async.js:93:12)
    at _drainQueue (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/async.js:86:9)
    at Async._drainQueues (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/async.js:102:5)
    at Immediate.Async.drainQueues [as _onImmediate] (/var/lib/ghost/versions/3.34.1/node_modules/bluebird/js/release/async.js:15:14)
    at processImmediate (internal/timers.js:456:21)

HTTPError: Response code 503 (Service Unavailable)
    at EventEmitter.<anonymous> (/var/lib/ghost/versions/3.34.1/node_modules/got/source/as-promise.js:74:19)

[2020-09-26 08:41:53] ERROR Response code 503 (Service Unavailable)

Response code 503 (Service Unavailable)

"The xmlrpc service was unable to send a ping request, your site will continue to function."
"If you get this error repeatedly, please seek help on https://ghost.org/docs/."

Error ID:
    207a5860-ffd4-11ea-8564-775b063361c8