For a while now I've been dealing with issues adding/updating events to my Google Calendar from Thunderbird/Lightning. When ever I'd try to update an event, I'd get a very generic "MODIFICATION_FAILED" error message with no detailed message. I could view events just fine and even dismiss alarms, but nothing I tried would allow me to add new events directly from Thunderbird. I ended up resorting to opening up Google Calendar and adding the events via the web interface. While this works, it's no where near as fast as accepting an invite from Thunderbird.
Today I had several GoToMeeting invites that I need to add to my calendar and decided look into the problem again. After doing some searching, I found lots of threads of people having the same issue, but alas no solutions. The one thing that did keep popping up is that Google considers CalDAV support still experimental, so problems may occur.
This got me to look into if there were other solutions other than CalDAV and I came across Provider for Google Calendar 0.7.1 add-on for Sunbird and Lightning. After changing all my calendars to use the new provider, everything seems to be working very well.
So, if you're getting errors updating your Google Calendar using CalDAV, you might try the Provider for Google Calendar to see if it fixes your issues. I'd also recommend taking a look at the Calendar:GDATA Provider wiki page which covers installation/setup of the provider.