Issues updating Notification from version 6.3.2 to version 7.0.3

Hello,

We are currently using WordPress 5.4.
We had the following plugins installed and worked perfectly:

Notification v6.3.2
Notification: Conditionals v1.1.0

We decided to update the Notification plugin to its latest version (7.0.3).
Installation was successful but activating the plugin will result in a so-called “white page of death”.

After deactivating the Notification plugin, everything gets back to normal.
Well, we had to move the plugin away from the plugins folder on the server, refresh the WP Plugins page, move it back and then refresh the WP Plugins page again. These steps force the plugin to become deactivated.

We noticed that there was also a newer version available for Notification: Conditionals plugin, so we updated it to version 1.2.0 to avoid compatibility issues.

Then we activated the Notification plugin again, but the “white page of death” showed-up.

We enabled some debugging and there are some Warnings that we can see:

Warning: ftp_nlist() expects parameter 1 to be resource, null given in /data/www/our_site/wordpress/wp-admin/includes/class-wp-filesystem-ftpext.php on line 401

Warning: ftp_pwd() expects parameter 1 to be resource, null given in /data/www/our_site/wordpress/wp-admin/includes/class-wp-filesystem-ftpext.php on line 225

Warning: ftp_pwd() expects parameter 1 to be resource, null given in /data/www/our_site/wordpress/wp-admin/includes/class-wp-filesystem-ftpext.php on line 225

If we install Notification v6.3.2, it works.

How can this issue be fixed?

Thank you,
Mihai