Note: The library has now been updated to 2.6.0 with this required change.
There is an important update to the Adafruit IO Arduino client library that will be released tomorrow. We will perform this update as a coordinated update with the SSL certificates for io.adafruit.com. The client library contains an SSL fingerprint that checks the validity of the connection.
You will need to download the library update, and then compile/upload to your devices once the update is released tomorrow. We will update this post once the update has been pushed.