As described in issue #1738560 plupload won't work if using the suhosin php extension in some cases. You can fix the problem by changing the suhosin configuration.

; in /etc/php.../suhosin.ini
; ...
; We need bigger values here because using the defaults stops plupload working.
suhosin.request.max_varname_length = 128
suhosin.post.max_name_length = 128

The patch adds this info to the installation notes in the readme file.

CommentFileSizeAuthor
plupload_suhosin_notes.patch847 bytesorganicwire