[O] problem with org-ref after upgrading org-mode from 8.3 to 9.0.5

[Johannes Brauer]

Hi,
after the upgrade (require 'org-ref) results in the error message:
 (error "Required feature ‘let-alist’ was not provided“)

I am using Aquamacs 3.3  GNU Emacs 25.1.1. I looked for let-plist using 
list-packages and found:
  let-alist          1.0.4         built-in 

 What is going wrong?

Johannes