It appears that the format of API help is changed from /api/help/ to /apiHelp. However, this change breaks old links (for example stored in the history, or linked from another sites). Also, some links on these pages are still not changed (for example, the Comment link at http://mirror.codeforces.com/apiHelp/methods#blogEntry.comments points to the broken link http://mirror.codeforces.com/api/help/objects#Comment.

Suggestion: make the old-style links automatically redirect to the new-style links.