curl reports <url> malformed, other RSS fetcher like TT-RSS work

[capricorn72]

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Explain the Problem

On some feeds, an error of curl seems to stop fetching feeds, but other RSS fetchers like Tiny Tiny RSS work well and even RSS validators say "all fine".

Steps to Reproduce

Explain what you did to encounter the issue
Add, for example, the RSS feed https://www.n-tv.de/politik/rss
An error no. 3 of curl pops up.

System Information

• News app version:15.3.2
• Nextcloud version: 20.0.6
• Cron type: (system cron/python updater/...) system cron
• PHP version: php7.4 7.4.15-7+ubuntu18.04.1+deb.sury.org+1
• Database and version: mysql-server 8.0.23-1ubuntu18.04
• Browser and version: Vivaldi | 3.6.2165.40 (Stable channel) (64-Bit); Chromium Version 88.0.4324.182 built on Debian 10.8, running on Debian 10.8 (64-Bit)
• OS and version: Debian 10.8 64

Contents of nextcloud/data/nextcloud.log

No entry when filtering to "n-tv"

Paste output here
Which output?

Contents of Browser Error Console

Read http://ggnome.com/wiki/Using_The_Browser_Error_Console if you are unsure what to put here

POST https://********************/index.php/apps/news/feeds 422 (Unprocessable Entity)
(anonymous) @ angular.min.js:114
y @ angular.min.js:109
(anonymous) @ angular.min.js:107
(anonymous) @ angular.min.js:141
$digest @ angular.min.js:153
$apply @ angular.min.js:156
(anonymous) @ angular.min.js:200
dispatch @ jquery.js:4737
v.handle @ jquery.js:4549
angular.min.js:129 TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at Object.app.service.publishAll (Publisher.js:34)
    at NavigationController.js:214
    at angular.min.js:141
    at c.$digest (angular.min.js:153)
    at c.$apply (angular.min.js:156)
    at angular.min.js:107
    at y (angular.min.js:113)
    at XMLHttpRequest.$.onload (angular.min.js:113) "Possibly unhandled rejection: {}"

[Grotax]

News can't handle the missing self link of the feed. It is used to point to the actual position of the feed, which is why news takes that and tries to connect. But this feed has <atom:link href="" rel="self" type="application/rss+xml"/> and so the curl error happens.