Ok, I started the container again with DEBUG=True as environment variable. I have now also explicitly specified a Mac address and IP address, although this should make no difference, both containers are located in Macvlan and thus act as independent hosts.
Unfortunately the output didn’t become more helpful with this.
192.168.178.5 - - [30/Mar/2020 20:31:33] "GET /deconz HTTP/1.1" 200 -
192.168.178.5 - - [30/Mar/2020 20:31:33] code 404, message not found
192.168.178.5 - - [30/Mar/2020 20:31:33] "GET /favicon.ico HTTP/1.1" 404 -
192.168.178.5 - - [30/Mar/2020 20:31:35] "GET /deconz? HTTP/1.1" 200 -
I now see a 404 notification, but honestly don’t know why, as both containers could definitely see each other as I said. Whether I call the diyhue page with “/deconz” or “/deconz?” makes no difference, it redirects me to “/deconz?” anyway.
Interestingly, I found out that I can also specify an IP address and port via “Import from Bridge”. With that the connection worked immediately 
But I guess this connection method is for original Hue Bridges? So I don’t know if there are any disadvantages in connection with Deconz?