"Do you have a lot of files (documents) in the My Documents Folder? Perhaps you have hit the limit of how many files are permitted in a folder."
There isn't a limit on how many files you can have in a folder, well there is but the number is enormous, in real life you'd never reach it. There is a 4GB filesize limit when using the FAT32 system but you'd know if that was the problem.
p;3: It's possible the file you are trying to edit is corrupt in some way, try copy and pasting the contents of that file into a new document and moving the problem file out of the My Documents folder, see if it helps.
What happens if you try to edit another file in the My Documents folder, do you get the same error?