I am running a docker based Home Assistant installation and I have installed Network UPS Tools (NUT) following the description given on YouTube by Homelab in “Network UPS Tools (NUT Server) Ultimate Guide” (only difference: I configured a standalone set-up). After the installation of NUT on the host I could read the values from the UPS via the upsc command and Home Assistant automatically recognized the server and offered an integration. When I clicked on “Configure” I only saw two data entry fields for “Username” and “Password”, but no matter what I tried I could not get it to work. I also could not click on “Add Integration” and manually select “Setup another instance of Network UPS Tools (NUT)”. When I did that simply nothing happened.
Finally I managed to get it to work by "Ignore"ing the offered default integration and then I was able to use the “Add Integration” functionality and select the NUT integration. After that I got a screen with 4 fields for the Host, Port, Username and Password. After setting Host to “localhost” and the port to 3493 and leaving username and password empty I got the integration to work.
I thought I’d document this behaviour here for others, because it took me quite some time to get it working.