I’m having the same problem here, running 3.5.2 and BP 1.7.3.
Default theme was installed and working. I’ve created a child theme with a style.css as per instructions linked above, then copied the header.php from the bp-default folder to the new theme folder, edited it to remove the search bar, activated the new child theme, everything was working fine. At next log-in the Dashboard says ‘ERROR: The themes directory is either empty or doesn’t exist. Please check your installation.’ and the Themes page ‘The parent theme is missing. Please install the “bp-default” parent theme.’
But all the bp-default files are still there on my FTP, no changes were made.
