The issue is a recurrent one, meaning Geary works just as expected to several months, then the notification pops up, I switch to BlueMail or MailSpring for a couple of months, then I notice Geary is back on its feet again and then again - it’s a vicious circle.
One thing is crucial here: outlook.com is always problematic and only outlook account.
Can it be fixed once and for all somehow?
If I recall correct - you need to enable support for external mail client if using outlook - perhaps you even need a paid account.
A possible alternative is to use the Thunderbird email client (with any service you choose). Even a sometimes frowned upon free email service such a Gmail could potentially be used, considering Thunderbird also incorporates end-to-end PGP encryption as an option.
sudo pacman -S thunderbird
Cheers.