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

;extension=iconv.so

is uncommented:

extension=iconv.so

Save, and if necessary, restart PHP-FPM.

I had this issue on Arch Linux, after the update to PHP 7.