I’ve found a recurring issue whenever I try to add a new feed to a folder that syncs with Feedly:
An error occurred while processing the “Feedly” account: The feed couldn’t be found and can’t be added. (Account.AccountError 1)
Please try again later.
The issue seems to only happen if the feed is unknown to Feedly.
I had this issue with https://guyenglish.net/feed.xml but was able to solve it by going to the Feedly website and subscribing manually. I suspect that going to the site to add it there actually added the feed to their system, because after I did that I was able to delete it and re-add it without any issues.
I just ran into this again with https://zachmil.es/rss/ and thought I might pass it along before I added it to Feedly so someone else could investigate.