Creating a new Website creates a default favicon.ico which is of size 256x256 (This was with Plesk Onyx). But the size should be 16x16.
Hope this is the correct categorie (means creating a new website)
I thought, oh I have to create my favicon in the size of 256x256. But this was not correct. So please, use the correct size.
I'm working with IrfanView when I'm checking, creating and changing ico's
Plesk does not create a favicon file when you add a domain or subdomain. Please check your software where and why such a file is created and in which format that is done.