Briefly unavailable for scheduled maintenance. Check back in a minute.

워드프레스 플러그인을 업데이트 할떄 

Briefly unavailable for scheduled maintenance. Check back in a minute.

라고 표시되는 에러가 발생하면 

생성된 .maintenance 파일을 서버에서 삭제하면 됩니다. 

https://codex.wordpress.org/FAQ_Troubleshooting#How_to_clear_the_.22Briefly_unavailable_for_scheduled_maintenance.22_message_after_doing_automatic_upgrade.3F

How to clear the "Briefly unavailable for scheduled maintenance" message after doing automatic upgrade?

As part of the automatic upgrade WordPress places a file named .maintenance in the blog base folder (folder that contains the wp-admin folder). If that file exists, then vistors will see the message Briefly unavailable for scheduled maintenance. Check back in a minute.

To stop that message from being displayed to vistors, just delete the .maintenance file. The automatic upgrade should be executed again, just in case it failed.

Note the core automatic upgrade feature was added with Version 2.7.

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