PHP Warning: Undefined array key 1 in class-ftp-pure.php

Hi there,

I have just installed a WordPress in our server, but everytime I try to delete or install a new plugin and put the data of our FTP (which outside WP works), it shows me this message:

Installation failed: PHP Warning: Undefined array key 1 in …\wp-admin\includes\class-ftp-pure.php on line 78

I do not have any clue how to fix it to be able to add new plugins. I could not add anything new so far.

Many thanks in advance.

Best regards