I'm just guessing, but i have 2 ideas.

1. In /.config/evolution/sources directory there are some files with "source" extension. I created a few additional offline calendars by producing new .source files. Maybe the online calendar properties are stored here too. So just delete the adequate source file.
2. Try System settings > Accounts. A list have to be there that contains your online calendars. You might delete them.

@mateo_salta
Thank you Mateo! What a great and simple solution. It solved my problem. And i hadn't noticed that reinstall all apps button before. I love this community, and this OS.

I'm just guessing, but i have 2 ideas.

1. In /.config/evolution/sources directory there are some files with "source" extension. I created a few additional offline calendars by producing new .source files. Maybe the online calendar properties are stored here too. So just delete the adequate source file.
2. Try System settings > Accounts. A list have to be there that contains your online calendars. You might delete them.

@mateo_salta
Thank you Mateo! What a great and simple solution. It solved my problem. And i hadn't noticed that reinstall all apps button before. I love this community, and this OS.

@unisuperbox
Many thanks Dalton, I get it. The device is on 64-bit image now, but a lot of apps don't work (like file manager, bluetooth file transfer, openstore, terminal etc.). I think i installed newly the 32-bit image long ago. May it be the reason? I'm afraid i would lose my contact list, my messages, the installed apps etc. if i re-install with wiping the user data. Is it possible to keep (or somehow save) the personal data and get the apps work also?

In channels.json the 16.04/community/fredldotme/devel channel has a hidden:true tag. Why? This channel disappeared from the list in ubports installer and from the channel settings list on my suzu (Xperia X ) device. Is there any way to install the 16.04/community/fredldotme/devel images on my Xperia X?