Recently, NextGen Gallery stopped working on sites that also had Contact Form 7 installed due a recent update in Contact Form 7. These are two very popular free plugins that are used on many websites.
I recently came across this post that explains a simple way to fix this error and it worked on this site, as well as other people’s sites:
http://wordpress.org/support/topic/contact-form-7-breaks-nextgen-gallery-flash-upload
Basically, here is the step by step breakdown of how to fix this issue:
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