I use the kde-apps 5.12.2 at this moment.
If I open the agenda in Kontact and try to insert a appointment, and then want a reminder, kontact complains about selecting a valid collection first. But there is no agenda to select from.
I think this started with 5.12.1, anyhow with 5.12.1 I had this problem also.
In the left corner below I cannot see any agenda also, like it was before.

But I really like to use the agenda but don't know how.

At settings->setup KOrganiser -> Agendas
I have already the agenda I had before.
I can restart the agenda and then is says it is ready, but nothing really changed.

I have tried to remove all agendas and start a new one, but I got the same error.

So eh... help please?

Ok, I used a KAlarm agenda and that does not work anymore.
Now I have setup a new agenda with an Ical file and now it works again.