Major Lightning blocker problem here: I am running Thunderbird 52.5.0 64-bit on Red Had Enterprise Linux 7.1, both are fully updated. Starting today, when I try to create or edit a calendar/event item, the "Edit item" window shows up but the whole window is filed with just this message: "XML Parsing Error: undefined entity Location: chrome://lightning/content/lightning-item-iframe.xul Line Number 115, Column 7: <button id="counter-proposal-btn" ------^" This is a major problem for me because I rely on the calendaring functions of Thunderbird and this error completely destroys it! How do I troubleshoot and fix this?? Thank you.