Where did you get 15.4.5 from? The only versions I see for Nextcloud 16 is 14.2.2 based on the official app store: https://apps.nextcloud.com/apps/news
Also I'm pretty sure you'll have to clean the database table as well after you downgrade as there might be some changes to it which is causing the issue.
Another alternative, for those who have a selffhosted nextcloud, is the news extension in nextcloud. This is what I used to move away from TT-RSS.
I am going to have a look at FreshRSS. Seems nice.