ESP8266, Arduino core v2.6.1, fairly simple MQTT code (mostly like the provided example) written for arduino-mqtt v2.4.3. Today I have updated the library to v2.4.5 and suddenly it cannot connect to the broker. No other change has been made.

Downgrading to v2.4.3 of the library fixes the issue.

MQTT broker is Mosquitto. No TLS involved.

When the library's connect() fails, the lastError() returns -9, which is "LWMQTT_MISSING_OR_WRONG_PACKET", I suppose.