With CKeditor's upload feature enabled, create a text block with a link. For the link, upload a file through the CKeditor dialog. Use a filename with space in it.
CKeditor will retain the space in the URL. The copy code in Lessons parses URLs in order to update the reference to the new site. The Java normalize function will produce an error when the URL has a space in it. The same issue occurs with comma. I haven't yet checked all the characters.
We'll fix it in CKeditor, but we may have existing sites that won't copy, so I'm considering fixing at least space in the Lessons copy code.