Have you ever updated this site before? That specific file has not been updated for a while

Yes I have. I used 3.5.1 version. When i switch off the FTP layer, it updates correctly, but when the FTP layer is on, I have got this error.

I can confirm that, had to switch off FTP layer, too, to avoid the error.

See #11086

I have the same issue whatever the method I use (direct, hybrid or ftp)

@jppjpp

I have the same issue whatever the method I use (direct, hybrid or ftp)

yes, that codes global config FTP-enable setting and not the setting for Joomla upgrade method

The fix exists in J3.6.1+, older Joomla versions do not get the fix automatically, but this prevents you from running Joomla upgrade (also setting error_reporting to 'none' has not effect on this one)

Simplest way out of this is (if you can edit the file)

• is to edit the file and apply the patch manually and then upgrading Joomla should work
• also other option is to run your site without FTP, but since you have enabled in global configuration, it means that you need it, so better edit the file

At about line 488 of libraries/joomla/filesystem/file.php (backup the file 1st)
find:

$ftp = JFTP::getInstance($FTPOptions['host'], $FTPOptions['port'], null, $FTPOptions['user'], $FTPOptions['pass']);

replace with: (if you get syntax error then restore the file and retry)

$ftp = JClientFtp::getInstance($FTPOptions['host'], $FTPOptions['port'], array(), $FTPOptions['user'], $FTPOptions['pass']);