I have had the same problem for many months. I can modify a recurring
event with my iPhone and have it show in Lightning as long as I "edit
all occurrences". If I edit just the one occurrence on the iPhone it
shows on the iPhone but doesn't update Lightning. If I edit a single
instance of a recurring event in Thunderbird/Lightning I get the error
and calendar becomes read-only. I didn't have the issue with Outlook.
Leon