I have a new 2.8.6 not an upgrade
But help, screen options, widgets, switch between visual and html, upload media don't work.
I checked this forum on these issues and...
I don't have any plugins installed
re-uploaded wp-admin and wp-includes folders
And use the default theme
Also tried re-running the upgrade via Admin/Tools/Upgrade.
Downloading update from http://wordpress.org/wordpress-2.8.6.zip.
Unpacking the update.
Incompatible archive: PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
What else can I try?