Might be a bit of an edge case and I'm not even sure how I made this happen BUT...
if the wp_links table has been deleted AND the
link_manager_enabled option is still TRUE, you get this error when you're trying to add a new rss feed.
Like I said, I'm not even sure how I managed to pull this off, I think I may have manually dropped the links table without knowing there was a link_manager_enabled setting (until trying to debug this issue).
In other fun news, setting the value of
link_manager_enabled manually to 0 from 1 does work in getting the links menu to disappear, however, re-loading the wp-admin will reset it back to 1.
--The only solution, as far as I could figure out, was to remove the link manager option entirely.--
Update: Nope, that didn't work either. So I'm back to not being able to add a feed.
Rather than checking for the option, it might be worth checking for the existence of the links table or adding that to the check.