Are you using the vanilla versions of both, Thunderbird and Lightning from Mozilla, or the Ubuntu provided version? In the latter case please contact Ubuntu. Note, you must not mix the Mozilla and Ubuntu versions.

I just noticed extension "Provider for Google Calendar 1.9". I don't recall installing or approving it.

You possibly did when setting up your (Google?) calendar. In that case disabling the add-on may be a bad idea.

Hello,

I installed them ages ago, I don't know whether they are "vanilla" or "ubuntu" versions. Is there a way to find out?

In any case, I just asked this question at askubuntu.com.

I installed them ages ago

So you installed them, then you should know, shouldn't you.

christ1 said

I installed them ages ago

So you installed them, then you should know, shouldn't you.

I am sure some people remember every detail about every program they installed 5 years ago. I am afraid I am not one of them...

Put another way, the question is whether you used the version of Thunderbird that came with Ubuntu, or did you go out of your way to download and install from the mozilla site?

If the former, you are using a build as set up and managed by the people at Ubuntu and they are the proper people to help you out.

In a general case, i would advise uninstalling Lightning then reinstalling it, at which point the correct version should be selected automatically for you. But if it comes from the Ubuntu repository the standard procedures don't apply. But I think you could reset to a normal state by removing Lightning and then removing and reinstalling Thunderbird. This should get you back into step with the current repository offerings. (Doing this should not affect your current settings or calendar data.)

Provider is probably automatically included for you as it makes setting up a Google Calendar very simple and straightforward.