One of the features in Content Mode (mode, in which all editable content on a page will have "Edit Content" button next to it) is adding links to other pages on website. Apparently this was broken at some point during 5.3.x version development, because this is working in 5.2.x:
This happens because in - INP-1298Getting issue details... STATUS we've moved code, that was replacing "&" with "&" from the "kApplication::HREF" method into actual "m_T" (or "m_Link") tag, that were needed it. The "fck:OnLoadCmsTree" event (used to provide data for that empty dropdown) was expecting "&" from the "kApplication::HREF" method and was just placing them into XML. Now that's not happening, which results in broken XML document.
Manually html-escape URL used in "fck:OnLoadCmsTree" event to avoid broken XML.