ntopng only works if I select WAN interface, timesout if trying to use LAN interface

[Marty 1]

I have a netgate 4100 and just installed ntopng.
If I set the interface - LAN by itself and then try to access ntopng, it times out

The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer’s network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the web.

If I select WAN by itself, I can connect to ntopng.
If I try WAN and LAN, it times out as above.

The system log shows the following when trying to access ntopng after setting LAN interface.
(ntopng), jid 0, uid 0: exited on signal 6 (core dumped)