Cannot Save Files

I am attempting to save files to a remote Synology NAS. The process has been working well for months.

Starting a few hours ago, I get an error saying the file is already present and do I wish to replace it? Even when it is a new file in a new folder. No matter which option I choose, it writes a 0kb file to the drive.

It turns out I can copy files to the main level of the NAS. And then move them to a subfolder. But, I cannot copy directly to a subfolder.