Apparently it was caused by the following custom function for org-export-before-parsing-hook that I had to remove the subtree with "beameronly" tag:

(defun as/delete-ignored-heading (backend)

    (unless (equal backend 'beamer)

      ; remove subtree with beameronly tag
      (org-map-entries
       (lambda ()
         (progn
           (org-narrow-to-subtree)
           (org-cut-subtree)
           )
         )
       "+beameronly" 'tree)
      )
)

(setq org-export-before-parsing-hook '(as/delete-ignored-heading))

What I did wrong to cause this error? It worked for sum subtrees but not for others.

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From: joon.ro@outlook.com
To: emacs-orgmode@gnu.org
Date: Sun, 26 Apr 2015 14:35:53 -0500
Subject: [O] export fails with "byte-code: Before first headline at position ..."