Got the following error (in console) after pressing "ACCEPT" in message view. Invitation received via IMAP - meeting it self was most probably already listed in exchange calendar store as well.
-----------
41c8bc00-ab44-4796-8123-9aa1c9ddb936/myCal: PS: ACCEPTED
Error: [Exception... "'[JavaScript Error: "this.getInvitedAttendee(aOldItem) is null" {file: "file:///home/holgerg/.thunderbird/xxxxxx.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/modules/calUtils.jsm -> file:///home/holgerg/.thunderbird<email address hidden>/js/calExchange.js" line: 503}]' when calling method: [calICalendar::modifyItem]" nsresult: "0x80570021 (NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS)" location: "JS frame :: file:///home/holgerg/.thunderbird/xxxxxx.default/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/modules/calItipUtils.jsm :: anonymous :: line 728" data: yes]
Could you post the event invitation mail? You might have to base64 decode the relevant MIME section first to remove sensitive information.