seo – GSC: Sitemap could not be retrieved

I'm trying to send a very simple sitemap (for testing only) to the Google Search Console, but I'm getting the following error message:

╔══════════════╤═════════╤══════════════╤═════════ ══╤══════════════════╤═════════════════╗
║ Sitemap │ Type │ Submitted │ Last read │ Status │ Found URLs ║
╠══════════════╪═════════╪══════════════╪═════════ ══╪══════════════════╪═════════════════╣
║ /sitemap.txt │ Unknown │ July 17, 2019 │ │ * Could not retrieve * │ 0 ║
╚══════════════╧═════════╧══════════════╧═════════ ══╧══════════════════╧═════════════════╝

If you click on it, an additional error message will be displayed: "(!) Sitemap could not be read".
However, clicking on "OPEN SITEMAP" will open it normally.

question
Any idea what's going on?


Domain: world-hello.ddns.net
Sitemap file: sitemap.txt
Server: Apache (Debian)