Using Octopus Deploy v3.1.4
I tried to create a feed but received the attached error. From a little trial-and-error it seems to be related to a limitation on feed name length.
On inspecting the database schema it looks like it (the database at least) should support a feed name of up to 200 characters. The feed name I am trying to create is 45 characters.
Octopus Deploy derives a dbo.Feed.Id (PK) value from the feed name. However, this is limited to 50 characters as shown. Therefore, in practice feeds are limited to having names of 44 characters, at least at the point of creation.
Please see screenshots.