Sorry to say, I still have the same issues and have tried a lot of things. The setup was - and is - identical to your description.
I suspect it is the Data To Server that is not working, because I can connect to the local broker from external clients and get the mosquitto $SYS topic update each 10 s. Data to Server to external brokers don't seem to be working either.
I was about to try to use CLI and try mosquitto sub/pub but was kicked out, this goes for telnet and ssh as well. Another wall!