The problem is the server can not connect to CODE component. There might be multiple issues I would start checking the list below. do you have separated CODE from aio or did you install build-in version (Could not establish connection to the Collabora Online server (Using "Collabora Online - Built-in CODE Server" option) - #2 by szaimen)
- in case of separated AiO Collabora your instance should be reachable at
https://nextcloud.greatday.fit/hosting/discoverywhich is not the case now. Please review your reverseproxy config - sometimes docker container can not talk to each other/itself using public DNS name, you might need to setup a DNS alias to short-cut the connection to the public-facing container like discussed here: Probably DNS help with NC Docker + Collabora + Wireguard tunnel - #5 by wwe
- in case you are using collabora-built-in it is hosted at
${server}/apps/richdocumentscode/proxy.php?req=so you should see XML describing CODE capabilities athttps://nextcloud.greatday.fit/apps/richdocumentscode/proxy.php?req=/hosting/discoverywhich is not reachable for me.
please look through other threads - this problem is very common for collabora-online and collabora-built-in installations.