This is marked as 'solved' as the post contains a work-around for 'not so technical' users; but IMHO a fix should be put into the Ubuntu software centre / auto-update process.

PC is running Ubuntu 11.10; with (originally) Thunderbird 14 & Lightning 1.6 installed & functioning (do not have precise details of the original installation process used); used mainly by 1 user on the machine with sudoer rights (a.n.other user has the 'admin role', but the TB user is aware of the admin password).

The Ubuntu auto-update pushed out early October upgraded TB to v15; requiring in turn an upgrade of Lightning to v 1.7, as TB v15 add-on-manager correctly reported that Lightning v1.6 was incompatible.

The add-on-manager duely found and downloaded Lightning v 1.7, and disabled v 1.6, but attempts to click on the 're-start TB now to install Lightning v1.7' link did not result in any change to the add-on manager screen; and the Lightning calendar was not available.

Another 'symptom' is that clicking on any of the add-on manager links related to version 1.7 display info related to v 1.6

WORKAROUND:

1. click the 'remove' button for Lightning v1.6 (this will not destroy your calendar data as per this post from the Tbird forum: http://forums.mozillazine.org/viewto...8431#p12348431 ) .

2. SHUTDOWN / exit Thunderbird.

3. Open a Terminal / console window (launcher bar: Dash Home, then 'more apps', 'filter apps', 'accessories', 'terminal' ).

4. Start Thunderbird as 'superuser' using the following commands typed into the Terminal window:

Code:
sudo ls
then enter YOUR password (this is assuming your machine is set up so that your logon account has sudoer rights).

After the output of the ls command is finished, type

Code:
sudo thunderbird &
Provided the Thunderbird command is entered soon enough after ls, you will not be asked for the password again (see below).

5. If the Lightning add-on is still visible in the add-on manager page, remove it.

6. Now use the add-on manager to navigate to the Mozilla hosted version of Lightning 1.7 and install it.

7. Your calendar should now be available with all the original data entered previously.

8. Exit Thunderbird

9. Close the terminal window by typing

Code:
exit
10. Re-start Thunderbird using the normal launcher icon, re-check that the calendar and previous items can be accessed.

END OF WORK-AROUND.

NOTES/ additional info:

a. if you try the 'sudo thunderbird &' command immediately after terminal window is opened, the password is requested, then 'nothing magic' happens: this is due to some sort of bug/issue with (presumably) the X-window manager that is involved with all the Thunderbird GUI displays.

b. The root cause of the issue is a file permissions issue due to the permissions assigned at the time Thunderbird & Lightning was originally installed vs file permissions generated by the Ubuntu Software centre auto-update process, as can be seen below:


Code:
:~/.thunderbird$ pwd
/home/charlie/.thunderbird
:~/.thunderbird$ ls -alrt ./xxz2zfdx.default/extensions
total 992
-rw-r--r--  1 root    root    564663 2012-07-17 13:23 tbtestpilot@labs.mozilla.com.xpi
-rw-r--r--  1 charlie charlie 178675 2012-10-04 22:39 {3c8e8390-2cf6-11d9-9669-0800200c9a66}.xpi
-rw-r--r--  1 charlie charlie 256607 2012-10-04 22:39 {58D4392A-842E-11DE-B51A-C7B855D89593}.xpi
drwxr-xr-x  7 root    root      4096 2012-10-05 14:40 {e2fda1a4-762b-4020-b5ad-a41df1933103}
drwxr-xr-x  3 charlie charlie   4096 2012-10-05 14:40 .
drwx------ 12 charlie charlie   4096 2012-10-07 09:33 ..
The .xpi files are the ones that were downloaded as a result of the initial upgrade to Lightning 1.7 attempt, when user 'charlie' had started Thunderbird normally.

The directory with 'root' permissions contains the Lightning installer from the Mozilla web site after starting Thunderbird via 'sudo thunderbird &'