When trying to upgrade WordPress to version 5.8 I encouraged a strange error:
Update WordPress
Downloading update from https://downloads.wordpress.org/release/wordpress-5.8-no-content.zip…
The authenticity of wordpress-5.8-no-content.zip could not be verified as no signature was found.
Unpacking the update…
Could not copy file.: wordpress/wp-includes/css/dist/edit-post/style-rtl.css
Installation failed.
This looks at a first glance like a timeout or permission error. The signature error is actually only a warning and doesn’t prevent the installation from continuing.
The real issue is actually that there is not enough space. Downloading the zip succeeds, but this uses up the whole quota which is left on the account, so copying the files fails as there is no space left.
Increasing the account quota fixed the issue. In case this is not an option for you, you can manually upgrade WordPress by downloading the zip to your machine, unpack it and then upload all files.