Agent stopped working with 503 error

Starting vault agent stopped working today with following messages.
It was all ok until today with exact same configuration.

==> Vault agent started! Log data will stream in below:

==> Vault agent configuration:

           Api Address 1: http://127.0.0.1:8201
           Api Address 2: http://bufconn
                     Cgo: disabled
               Log Level: info
                 Version: Vault v1.12.3, built 2023-02-02T09:07:27Z
             Version Sha: 209b3dd99fe8ca320340d08c70cff5f620261f9b

2023-02-28T17:22:58.173+0900 [INFO]  sink.file: creating file sink
2023-02-28T17:22:58.173+0900 [INFO]  sink.file: file sink configured: path=./.token mode=-rw-------
2023-02-28T17:22:58.174+0900 [INFO]  sink.server: starting sink server
2023-02-28T17:22:58.174+0900 [INFO]  template.server: starting template server
2023-02-28T17:22:58.174+0900 [INFO]  auth.handler: starting auth handler
2023-02-28T17:22:58.174+0900 [INFO]  auth.handler: authenticating
2023-02-28T17:22:58.174+0900 [INFO] (runner) creating new runner (dry: false, once: false)
2023-02-28T17:22:58.174+0900 [INFO] (runner) creating watcher
2023-02-28T17:22:58.425+0900 [ERROR] auth.handler: error authenticating:
  error=
  | Error making API request.
  |
  | URL: PUT https://vault-dev.nubison.io/v1/auth/approle/login
  | Code: 503. Raw Message:
  |
  | Backend action does not exist
   backoff=1s
2023-02-28T17:22:59.426+0900 [INFO]  auth.handler: authenticating
2023-02-28T17:22:59.502+0900 [ERROR] auth.handler: error authenticating:
  error=
  | Error making API request.
  |
  | URL: PUT https://vault-dev.nubison.io/v1/auth/approle/login
  | Code: 503. Raw Message:
  |
  | Backend action does not exist
   backoff=1.52s
^C==> Vault agent shutdown triggered

The vault agent deployed in k8s still works fine. I can log into the vault server and found no changes at all.

What can I do to solve this?