From old forum:

mc-abeAdmin
We enabled use of the DNS server in the configuration of the gateway in version 0.6-355. Do you have a DNS server in there that cannot resolve your MQTT broker name? I explicitly tested the MQTT functionality before releasing version 0.6-355. I will do a test again and see if I have different results.Like

kristofferis
Iam using IP address so there is no issue with DNS, I noticed in the release notes that in 0.6-355 you enabled support for ip address, but that was working in 0.6-354.