Set assets.export_concurrent to false
Set assets.export_concurrent to false

There seems to be issues with Sprockets version 4, errors like [1]
occur when precompiling assets.

A suggestion I've seen to avoid this is to add this configuration [2].

1: SassC::SyntaxError: Error: error in C function font-path: undefined
method `[]' for nil:NilClass
2: https://github.com/rails/sprockets/issues/581#issuecomment-486984663

alphagov/contacts-admin,alphagov/contacts-admin,alphagov/contacts-admin