Akregator stopped fetching some of my feed subscriptions a while ago (sorry, I don't have the exact date). The problem seems to be related to feeds hosted at tumblr.com.

How to reproduce:
Add a tumblr.com feed such as https://moviecode.tumblr.com/rss to Akregator. Akregator says "Feed not found from https://moviecode.tumblr.com/rss. The same link works in Firefox as a Live Bookmark though.

I'm running Akregator 5.7.3 on Fedora 28 KDE Spin.

Just ran into this frustrating problem that only seems to affect EU states.

this appears to be a GDPR-related problem, Tumblr is trying to issue some sort of consent form and Akregator doesn't know what to do with it so the RSS comes out as invalid.

source, also here