I ran into the very same issue - zero bytes free on my WDMC. If you can access your WDMC via ssh, try this - assuming the file you want to delete is file.xyz:
> file.xyz
this deletes just the contents of the file. Now you should have some space left again. Start with small files first. Worked for me.
In my browser it shows correctly, it’s > file.xyz or, formatted as html, > file.xyz - this will empty an existing file. It’s basically the same as echo “” > file.xyz. At least in bash you can skip the echo “” part, it’s valid.