Kibana not loading properly. Check the server output for more information

I have Elasticsearch 7.2 and Kibana 7.2 on a RedHat 7.4 zLinux. I can access the Kibana from any host on the same VLAN, either with port 5601 or nginx port 9000, but when I try to access from outside the VLAN it fails with 'Kibana did not load properly..' and Firefox 60.8.0esr gives the following in debug:
Loading failed for the with source “http://xx.xx.xx.xx:5601/built_assets/dlls/vendors.bundle.dll.js”.
The networking people say there is nothing wrong with the network. Ports are open.
I have tried with csp.strict: true/false but no difference.
After reading this forum I suspect the vendors.bundle.dll.js is truncated but how can I fix that?
Thanks
Peter