I further tried to narrow down what I’ve written in this topic and came to the conclusion that the youtube video fetcher really seems to have a bug.
Please see the following opml: all the RSS feeds are valid and perfectly shown in the browser. I can also import them without problems in other readers (feedly, inoreader). But for whatever reason Newsblur says that the feed are broken.
<?xml version="1.0" encoding="UTF-8"?>
Youtube feeds that are not correctly imported in Newsblur
https://www.youtube.com/feeds/videos.xml?channel_id=UCLNVO6TQ8vUGiSgVc-7hWiQ"; htmlUrl="" rel=“nofollow”>https://www.youtube.com/channel/UCLNVO6TQ8vUGiSgVc-7hWiQ"/>;
https://www.youtube.com/feeds/videos.xml?channel_id=UCQUUysbbowBlnmL4XF5fQQw"; htmlUrl="" rel=“nofollow”>https://www.youtube.com/channel/UCQUUysbbowBlnmL4XF5fQQw"/>;
https://www.youtube.com/feeds/videos.xml?channel_id=UC09fqoXU9moUocftVLLTctA"; htmlUrl="" rel=“nofollow” title=“Link: https://www.youtube.com/channel/UC09fqoXU9moUocftVLLTctA"/>">https://www.youtube.com/channel/UC09fqoXU9moUocftVLLTctA”/>;
https://www.youtube.com/feeds/videos.xml?channel_id=UCCAuVN4JcLYNaG7b-XEPaYw"; htmlUrl="" rel=“nofollow” title=“Link: https://www.youtube.com/channel/UCCAuVN4JcLYNaG7b-XEPaYw"/>">https://www.youtube.com/channel/UCCAuVN4JcLYNaG7b-XEPaYw”/>;
https://www.youtube.com/feeds/videos.xml?channel_id=UCiioH25tZ_w447aGN9fRb0g"; htmlUrl="" rel=“nofollow” title=“Link: https://www.youtube.com/channel/UCiioH25tZ_w447aGN9fRb0g"/>">https://www.youtube.com/channel/UCiioH25tZ_w447aGN9fRb0g”/>;