If you’re seeing this when running apt update on your UniFi controller, then run the code at the end of this post.
An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release: The following signatures were invalid: EXPKEYSIG BC711F9BA15703C6 MongoDB 3.4 Release Signing Key
W: Failed to fetch http://repo.mongodb.org/apt/ubuntu/dists/xenial/mongodb-org/3.4/Release.gpg The following signatures were invalid: EXPKEYSIG BC711F9BA15703C6 MongoDB 3.4 Release Signing Key W: Some index files failed to download. They have been ignored, or old ones used instead.
wget -qO - https://www.mongodb.org/static/pgp/server-3.4.asc | sudo apt-key add -Source: https://www.reddit.com/r/Ubiquiti/comments/eky995/psa_for_those_with_unifi_controllers_w_mongodb_34/?utm_source=share&utm_medium=web2x