Hello,
if you don't have an Fusion capable (ex. Seven, Drupal default) theme enabled as default this could make some problems, in my case the website displays incorrect under ie7/8. After disabling this line in file fusion_responsive_ie.module, it worked as it should:

if (theme_get_setting('responsive_enabled', $default_theme) && theme_get_setting('responsive_ie_shim', $default_theme))

I use the Acquia Marina theme with themekey module in a Multisite-Drupal Setup....

greetings
chris

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