Soup maker vegetable soup.

I use a pot, have no patience to machines - too confusing.

Blame the machine!

My guess is the stock cube. I don't use stock cubes because of possible ingredients, knowing I can do better on my own.

Sometimes I have to add a bit of sugar to a curry or tomato sauce to adjust seasonings - some herbs can tend bitter in quantity.

Celery can be bitter as well. Again, a bit of sugar/sweetner/ honey (any form) would be my solution.

I made cucumber soup last year with a glut of cucumbers - it was the vilest soup I've ever made!
Very bitter and, after all that effort, it went down the loo.

I would think it could be the courgette too.

If I'm making soup in the soup maker, I always cook the onion a little first, either fry it gently or in some of ths stock in the microwave.

I cannot think why a soup made with these vegetables could taste bitter.

The only long shot I can think of is that the courgette was bitter with a high level of curcurbitin in it. This is not common but the recommendation in that case is not to eat it
Here is a reference www.dtbrownseeds.co.uk/Gardening-Advice/Courgette-Bitterness-DTB/

Assuming that is the cause, do not give up heart and try another mixed veg soup, but without courgette.