Since the SSL fingerprint of AdafruitIO has been changed as shown in the 86733a8 commit of Adafruit_IO_Arduino repo the adafruitio_esp8266_sicure sketch should be updated:
const char *fingerprint = "AD 4B 64 B3 67 40 B5 FC 0E 51 9B BD 25 E9 7F 88 B6 2A A3 5B";
https://github.com/adafruit/Adafruit_MQTT_Library/blob/1e3fb01778ef9eddcf68b5ac5f8a1cd6ec923a81/examples/adafruitio_secure_esp8266/adafruitio_secure_esp8266.ino#L44
Since the SSL fingerprint of AdafruitIO has been changed as shown in the 86733a8 commit of Adafruit_IO_Arduino repo the adafruitio_esp8266_sicure sketch should be updated:
const char *fingerprint = "AD 4B 64 B3 67 40 B5 FC 0E 51 9B BD 25 E9 7F 88 B6 2A A3 5B";
Best regards, Biagio