Steps to reproduce the issue

a customer of mine reports a PHP Warning:

PHP Warning: fopen(): Unable to find the wrapper "buffer" - did you forget to enable it when you configured PHP?

in

/libraries/src/Client/FtpClient.php

joomla 3.8.1 + php-fpm 7.0.22 via proxy_fcgi

it seems that:

$buffer = fopen('buffer://tmp', 'r');

the buffer may not be always registered type for streams. (actually in most cases it probably is not)

Registered PHP Streams (defaults i believe):
php5.6 to php7.2:
https, ftps, compress.zlib, php, file, glob, data, http, ftp, phar, zip

best regards, stan

Expected result

Actual result

System information (as much as possible)

Additional comments