Have you tried:
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deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
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switching to the Twenty Eleven theme to rule out any theme-specific problems.
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resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
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re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.
Tip from a thread in WordPress forums.