Edit : I replaced the port number in my Yunohost-config ( proxy_pass http://127.0.0.1:12716;) with the one in the Github wiki (3001) ; now I get a friendly Yunohost notice of a naughty gateway,
When I revert to the original configuration (12716), and add
proxy_set_header Host $host;
test the config, restart nginx and visit the site… Nothing happens, for quite a while. Then I get the red bar with the error notice 
, but THEN after a couple of seconds, it continues on to the login page of Uptime Kuma! 
Login does not seem to be integrated into SSO, also not after re-login to Yunohost. Perhaps as a result of the proxy_set_header Host $host ?
Anyway, I can start adding monitors.
Would you have another suggestion for the nginx-configuration ?