Try another broker alias (don’t use the FQDN since the dots may? cause the problem. If(?) the dots cause the problem, report here so we can either modify the docs to raise caution (not to use dots) or to raise a PR against the binding to allow dots in the broker alias.
mqtt.cfg
adafruit.url=ssl://io.adafruit.com:8883
adafruit.clientId=<my key here>
...
Additionally, try to re-install the MQTT Action Add-On and check the contents of /var/lib/openhab2/config/org/openhab/mqtt.config
Also: It’s much better to start a new thread and not to reply to a resolved case since not many people will read it 