Still no success, but please see my comments inline below:
I decided to bite the bullet and used my office calendar via its web interface to book a Google calendar. It works, but there are a few pitfalls along the way. If everything is set up correctly, then it works. Here are a few points to check.
1. My Google calendars are accessed via "Provider for Google Calendar 3.1". I don't think it is currently possible to use Google calendars otherwise (it used to be possible a few years ago). If you have set up a calendar as Ical, the calendar is set, but it will fail as soon as you want to modify it in Thunderbird/lightning.
I am using Provider 3.1
I am trying without success to use an iCal calendar newly set up in TBird. It keeps reverting to read only.
Are you suggesting that I use iCal, or another calendar option in TBird? If so, which option? Also, should I be using the Google iCal location, or the Google HTML location, or something else?
2. With Provider for Google Calendar, connect to your Google account and then to the calendars you want to use.
3. For each of your calendars (Google or otherwise), right-click on the calendar name and select "Properties".
In the properties, check your "email address" (second to last line – see note at the end).
I think that you can only accept events in calendars for which you have set an email address, as well as local calendars.
I have entered my non-google email address.
4. When someone invites you to an event, the blue line "Accept" appears. If you click to accept an event, then all calendars for which you have set an email address will be listed.
Sorry, but I don't understand your comment #4 above. Where would these calendars be listed?
5. Once you have selected a calendar, the event will be added to that calendar. An email confirmation may also be sent to the meeting organizer, depending on settings in about:config.
a. Some Google calendars don't have any "email address" settings. I have also read in the WIKI that only the primary calendar of an account can be configured to accept invites.
I have entered my non-google email address in the settings for what I now understand to be my "primary" calendar.
b. To enable automatic receipts when you accept and event or modify its details, visit the Provider's wiki page, and also read the accompanying warning.
I have turned on calendar.debug.logging and when I try to accept an invitation, the log shows this error[color=#FF0000]: Timestamp: 2/16/2017 8:46:20 PM
Error: An error occurred when writing to the calendar KSA-OLD! Error code: MODIFICATION_FAILED. Description:
Source File: resource://calendar/modules/calUtils.jsm -> file:///F:/Thunderbird/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/calendar-js/calCalendarManager.js
Might you have an understanding of the meaning of that error?
Thanks for any further help on this,