This bug was first posted here: http://lists.typo3.org/pipermail/typo3-english/2012-March/079796.html
I have got the same on a 4.5.19.
In function indexAction(), in typo3/sysext/workspaces/classes/controller/PreviewController.php, labels are not escaped for 'visualPreview' .... 'modeHbox'.
These labels should go through addInlineLanguageLabel or equivalent from the class t3lib_pageRenderer.
I will try to submit a patch later.