If you are not already using the latest version of WP-Juicebox (v1.5.0.1), then please try upgrading.
You can download the latest version from the plugin's support page here.
After upgrading, please clear your browser's cache (to ensure that no older cached files are being used) and try again.
If you are already using the latest version, then this error message is most often associated with the use of a third-party editor (rather than the default CKEditor which comes bundled with WordPress). If you are using a third-party editor, try temporarily disabling it to see if this helps.
The next most likely cause might be whitespace being introduced by a core WordPress file.
If you have manually edited any core WordPress files (such as 'wp-config.php'), make sure that you have not introduced any whitespace (e.g. line feeds or carriage returns at the end of the file outside the PHP section) that would be output when the file is parsed.
I hope these suggestions help.
However, if you continue to experience difficulties, please let me know and I'll try to help further.
Thank you.