That is so weird, and it's not the first time it's happened that someone hasn't been able to rename a file. I can't figure out why for the life of me-- I can open them and rename them just fine. Are you running as administrator on your computer? How deeply subfoldered are the files? I find that Downloads > Genetics Hair by Hat > [Almighty Hat] Silly Title (Relevant mesh and texture info, Pooklet'd) isn't too long for the game to parse, but that one got a longer folder name than most.

Try shortening the name of the folder as much as you can-- if you can't rename the folder, make a new one with a shorter name and just move the necessary files over to it. (I use HP Roman Model T (Low Maintenance) for mine, but it's in my Genetics Hair by Hat folder, which only exists so it's easier to keep track of what I've done.)

Do you have a whole lot of downloads? You may have run up against a file limit or something...