FTP method for plugin update: Errors and failure

I moved WordPress from a Fast-CGI environment to a hoster who runs PHP as an Apache module (mod-php or , so I’m battling file permissions left and right. Putting define('FS_METHOD', 'ftpsockets'); and my FTP credentials in wp-config.php smoothed out some operations, but other operations (esp. the Media Library and theme file editing) don’t even use the FTP method, so that’s still a challenge. But just now I discovered that an operation I thought was working isn’t. I updated themes successfully, but now I have a plugin that wants an update, and I got this instead of success:

Notice: Undefined variable: arg in /my/path/wp-admin/includes/class-ftp.php on line 508

Downloading update from https://connect.advancedcustomfields.com[…etc…%5D…

Unpacking the update…

Notice: Undefined variable: arg in /my/path/wp-admin/includes/class-ftp.php on line 508

Notice: Undefined variable: arg in /my/path/wp-admin/includes/class-ftp.php on line 508

Notice: Undefined variable: arg in /my/path/wp-admin/includes/class-ftp.php on line 508

Could not create directory.

Any ideas on what might be causing this? I don’t think it’s specific to the plugin, because the first error is before it even gets the package file.

And if you have any clever ideas about how to handle the uploads/ and themes/ folders, I’m all ears. Every security-conscious fiber of my body resists the ideas of widening permissions to 777 or changing the owner to be the webserver’s user, but I can’t think of any other recourse.