Finnair is driving me mad. Fare keep changing when I log and can't purchase

Long story short it has been one week since I started trying to book a flight from Bologna to Tokyo for my upcoming long trip in September. 23th of September to 16th of December to be precise.

When I'm not logged in a roundtrip fare for my preferred dates is around 800€, no long layovers or overnights like the one they sell with a stop in London.

If I try to buy the ticket the website doesn't let me in the payment page. And show a big: "Sorry!Something went wrong".

If I log in the fare doesn't exist anymore and the ticket price is now over 1000€.

I called customer service and they are not able to tell me what is going on.

Can someone replicate this assurance? It's driving me nuts.