eXist version and full stack from exist.log? 

2011/9/23 Вячеслав Седов <schematronic@gmail.com>
sometime eXist just "lost" it and produce error

HTTP ERROR 400

Problem accessing /exist/rest/db/upload.xq. Reason:

   Failed to read query from /db/upload.xq
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i don`t know what can lead to this

--
Dmitriy Shabanov