Editor stuck on loading (wordpress 6.6.2) standard theme

This is a fresh install of wordpress 6.6.2 with standard “Twenty Twenty-Four” version 1.2 theme. No plug-in installed.

The editor is stuck at 25% loading, same as in Editor doesn’t properly load | www.ads-software.com. I am selfhosting the site on my Synology NAS with PHP 8.2 and GNIX server (Webstation).

The following errors are shown in the developer console of Edge (also on Firefox and on different devices):

GET https://tech.inhortum.org/wp-json/wp/v2/users/me?context=edit&_locale=user 404 (Not Found)
OPTIONS https://tech.inhortum.org/wp-json/wp/v2/settings?_locale=user 405 (Method Not Allowed)
GET https://tech.inhortum.org/wp-json/?_fields=description%2Cgmt_offset%2Chome%2Cname%2Csite_icon%2Csite_icon_url%2Csite_logo%2Ctimezone_string%2Curl&_locale=user 404 (Not Found)|
GET https://tech.inhortum.org/wp-json/?_fields=description%2Cgmt_offset%2Chome%2Cname%2Csite_icon%2Csite_icon_url%2Csite_logo%2Ctimezone_string%2Curl&_locale=user 404 (Not Found)
GET https://tech.inhortum.org/wp-json/wp/v2/global-styles/themes/twentytwentyfour/variations?_locale=user 404 (Not Found)
GET https://tech.inhortum.org/wp-json/wp/v2/types?context=view&_locale=user 404 (Not Found)
OPTIONS https://tech.inhortum.org/wp-json/wp/v2/settings?_locale=user 405 (Method Not Allowed)
GET https://tech.inhortum.org/wp-json/wp/v2/types?context=view&_locale=user 404 (Not Found)

And perhaps the more serious one, but not sure:

site-editor.php:1 Uncaught (in promise) {code: ‘invalid_json’, message: ‘The response is not a valid JSON response.’}
Promise.then
(anonymous) @ data.js?ver=6f178ef23ef546c51cae:1650
(anonymous) @ data.js?ver=6f178ef23ef546c51cae:1700
(anonymous) @ data.js?ver=6f178ef23ef546c51cae:2721
(anonymous) @ site-editor.php:2255
(anonymous) @ site-editor.php:2256
edit-site.js?ver=387fc31d605a5a5d2257:40629 Warning: You are importing createRoot from “react-dom” which is not supported. You should instead import it from “react-dom/client”.
printWarning @ react-dom.development.js:86
error @ react-dom.development.js:60
createRoot$1 @ react-dom.development.js:29846
initializeEditor @ edit-site.js?ver=387fc31d605a5a5d2257:40629
(anonymous) @ site-editor.php:2283
Show 3 more frames
Show less
api-fetch.js?ver=eb5e80d6477f0bc94063:691

So, fresh install, self hosted with synology NAS, no plugins installed, standard theme.

I saw the comments that the ISP might need to do some adjusting, if you could explain what adjusting I might be able to resolve it on my NAS.