Hi José,
thanks for your posting!
You are right, there is a problem with the access to the Feedbooks Profile. It occurs because Feedbooks doesn't send a login page, but uses webserver authentication. I have to check, how I can fix this.
As a workaround you could do the following: before accessing the profile from the EPUBReader OPDS reader, open this URL:
https://www.feedbooks.com/user/profile.atom and login. Afterwards it should be possible to access the profile from EPUBReader.
Michael