Preview of pages/articles not working anymore

I have a WP Multisite network, and for every single site, in every single page or article, when trying to access preview from the “preview changes” button, I get the following error:

“You do not have permission to preview drafts”

Also, admin bar is only showing up for Home pages (i.e. the “edit page” button also doesn’t work)

I have tried to disable all plugins and change themes, but this is happening on more that 25 sites on the network…

thanks for any help!

Enrico