This is what it says, in a pop up window with a big red cross:
Error copying file or folder (this is the pop up heading)
Cannot copy my_big_file_name_here: The file name you specified is not valid or too long. Specify a different file name.
There's nothing different about the file name - just letters and numbers, same as all my others.
EDIT:
Oh, but you said the whole PATH can't exceed 255 characters, which it probably does. So, ooops.
My bad. I'll have to change it then I think.