calendar – Thunderbird do not show events any more

I have recently installed ubuntu 20.04 and started using thunderbird. At first when an invitation appeared I was prompted to add a calendar (do not remember exactly which one) and it seems that thunderbird supported events flawlessly. But today I realized that the events are not showing in my calendar or event panel.
For example I have an invitation in an email which when opened now it states that:

This message contains an event that already has been processed.

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but no actual event is shown on the panes.

I can see that a past event is shown in my pane though, if this gives any clue about the situation. Also, I have installed two addons in thunderbird:

  1. EDS calendar integration
  2. Lightning calendar tabs

but did not help either.

So, what should I do to being able to see my events? Any ideas?

Thunderbird version is 87.8.1.