There appears to be little information on the internet about this, so I'm recording it here so that it might help someone.
If you get a message from PHP that looks like this:
stream_filter_prepend(): unable to create or locate filter "convert.iconv.ASCII/utf8"
Then you need to open
/etc/php/php.ini and make sure the line
Save, and if necessary, restart PHP-FPM.
I had this issue on Arch Linux, after the update to PHP 7.