Activities Sync - Meetings, Task and Calls

At least two of our users, when trying to accept calendar invites from external sources via Thunderbird Lightning, have been unable to process the invites. This seems to be an intermittent issue, per the user comments below: "I was unable to add external event into my CRM calendar. I tried again last night and an individual event that would not save earlier was added to the CRM calendar but another recurring meeting still would not work. (Error message "Processing message failed. Status: 80004005")." And from another user "I noticed it happened with two meetings this week" I have a screengrab, but I get an "Error invalid backend response" from the sugaroutfitters system when I try to add an image to the ticket.