Composer unable to access d.o.project

I have previously been happily installing diff with composer require drupal/diff:8.x-1.x-dev and keeping my code base up-to-date with composer but now a composer update returns the following:

[RuntimeException]                                                                                         
  Failed to execute git clone --no-checkout 'https://git.drupal.org/project/diff' 'web/modules/contrib/diff  
  ' && cd 'web/modules/contrib/diff' && git remote add composer 'https://git.drupal.org/project/diff' && gi  
  t fetch composer                                                                                           
  Cloning into 'web/modules/contrib/diff'...                                                                 
  fatal: unable to access 'https://git.drupal.org/project/diff/': The requested URL returned error: 504 

What's different?

• Is there a missing composer.json in the diff package or something?
• Do we need an OAuth token for d.o. as well as one for GitHub?