Ok, it's a hosting issue. I have no idea what, but your server is no longer writing file contents, or creating new directories.
I debugged it back to @mk_dir() and this is failing.
Your access_log and error_logs are both 0 bytes for today. Directories are not being written, and files are being written with 0 bytes.
There really is nothing more we can do. The native PHP command mk_dir() is throwing an error.