Additional features, settings, or issues not covered by specific Microsoft Teams categories
Same issue here!
In Old Teams classic version, in Teams group (not channel), adding sharepoint site by Website app is working (get login screen and after login The sharepoint site loaded)
In Teams (new) give this error (before I get a login screen): You're viewing content from https://.....sharepoint.com/si.... Make sure you trust this site before using it. If the site doesn’t load properly, open it in a browser.
Any idea when it will be fixed in the new version?