Did you try the "Reset Network Access List Button" in the web admin and rebooting?
If that doesn't work, use a freeware application such as Notepad++ to create a file named "fun_plug" in the root of Volume_1 with the following line:
chmod 777 "/mnt/HD_a2/path/to/file.ext"
Change /path/to/file.ext to where your file is within Volume_1.
Save the file in Unix format (LF line endings) and reboot the NAS.
Try to remove the file again. Then remove the fun_plug file you created.
I have no idea why file(s) created on the NAS should suddenly have incorrect permissions. All files created via SMB have full read-write access to any user defined on the NAS.