User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:188.8.131.52) Gecko/20100401 Firefox/3.6.3 GTB7.0 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:184.108.40.206) Gecko/20100512 Lightning/1.0b1 Thunderbird/3.0.5 Sometimes when I create events in Lightning, I get a message saying that Lightning couldn't write to the calendar name (insert calendar name here.) The only details of the error message is "modification_failed" Reproducible: Sometimes Steps to Reproduce: It's completely random as far as I can tell. Expected Results: Created the event!
Do you have any calendars other than Google? If you do, does the type of calendar (ics/iCAL, CalDAV, Google, mozStorage) make a difference?
I experience this often with my (only) Google Calendar. Extremely annoying.
I have the same problem with Thunderbird v3.1.10 + "Provider for google calendar" v0.7 add-on. Besides this is very annoying and should be fixed hopefully soon, it would be helpful to let at least the "create event" window reappear, thus I could save my data.
I just got the following Fedora report https://bugzilla.redhat.com/show_bug.cgi?id=770857 I can reproduce with using the Google ICS URL and specifying iCalendar when creating the calendary. If I specify the type as Google and use the XML URL it seems to work fine.
There has been a major rewrite of the Provider for Google Calendar between version 0.32 and 1.0. A vast number of bugs have been fixed during this rewrite, therefore I am closing lots of old bugs that I think might either be fixed or no longer apply to the latest version. Please read the updated FAQ  for details on known issues. If you can reproduce your issue with the latest version of the Provider for Google Calendar and you can't find an existing bug that handles your case, please reopen this issue. Thank you for your understanding.  https://wiki.mozilla.org/Calendar:GDATA_Provider