Updating any plugins results in 0 Kb files

When I update any plugin through WordPress, it gives me an error like this:

The plugin gravityforms/gravityforms.php has been deactivated due to an error: The plugin does not have a valid header.

I’ve checked with my host and they say the ftp uploader in WordPress 5.7.2 is buggy. It downloads and extracts the update files into a temp directory just fine, but when it tries to move them to their actual directory, they all end up as 0 Kb.

My theme is currently Avada 7.4.1, and my PHP version is 7.40.2. In my wp-config.php file, I added define('FS_METHOD', 'ftpext'); many years ago to force it to require FTP credentials when doing any kind of updates.

Is anyone else having an issue like this? It seems unlikely that WordPress would have a bug like this, let alone that no one would seem to be talking about it. Maybe that FS_METHOD line is messing things up? Is there some other way to make it ask for FTP credentials that I can try?