Well, I've got some information why LJ can't create the feed, but it's weird...
This error message is occurring because the site in question is using the Wordpress Bad Behavior plugin, which blocks LiveJournal's servers from retrieving the syndicated feed.
Please contact the maintainers of the site and ask them to review their security settings and, if possible, temporarily disable the Bad Behavior plugin or add LiveJournal's IP address (204.9.177.18) to the plugin's white list. Once the configuration has been updated, you may try adding the feed again.
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This error message is occurring because the site in question is using the Wordpress Bad Behavior plugin, which blocks LiveJournal's servers from retrieving the syndicated feed.
Please contact the maintainers of the site and ask them to review their security settings and, if possible, temporarily disable the Bad Behavior plugin or add LiveJournal's IP address (204.9.177.18) to the plugin's white list. Once the configuration has been updated, you may try adding the feed again.
As to why Emcit worked even though it was using Bad Behavior too? Well, it apparently only is a problem when creating the feed. Once the feed exists, Bad Behavior doesn't block the retrieval. Very strange.
So one solution: adding 204.9.177.18 to BB's whitelist will solve the problem.