When executing pod update or pod install with Cocoapods 1.8.5, the error message is as follows：
Update all pods
Updating local specs repositories
[!] CDN: trunk URL couldn't be downloaded: https://raw.githubusercontent.com/CocoaPods/Specs/master/Specs/ Response: 400
Solution:
Follow the official documentation to add a source to the Podfile:
# Uncomment the next line to define a global platform for your project
source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '10.0'
... ...
... ...