“File type not allowed” error message when trying to edit/upload ANY file

This issue came up when I used Filester for the first time in months. Every time I attempt to edit or upload ANY type of file, I get the error “File type not allowed”. I’m accessing my server from my Administrator account, and I’ve never run into this problem before.

I also did troubleshooting with plugins—even with all other plugins disabled, the issue persists. I can access my server and edit/upload any type of file just fine using a SFTP client. It’s just Filester/File Manager.

Can anyone help? Thanks in advance.