Cannot launch an Anaconda Notebook

Since this morning I have been unable to launch an Anaconda Notebook from Anaconda Navigator. I received the following message:

「このページは動作していませんnb.anaconda.cloud では現在このリクエストを処理できません。
HTTP ERROR 500」

(translated by me: 'This page is not working nb.anaconda.cloud is currently unable to process this request.
HTTP ERROR 500")