Today I had the case that the following message suddenly appeared during an update:
The following signatures were invalid: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key
this was the solution,
1) delete old key:
# apt-key del B188E2B695BD4743
2) import new key:
# curl -sSL -o /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
3) update package list:
# apt update
The error should be fixed.
Thanks to https://www.myprohost.com/en/blog/apt-key-expired-debsuryorg