I downloaded 2.0.4 Now when I save a sketch I get the message that the folder name is invalid.
FMy folder name has a letter at the start but the message says I need a letter
Whats going on?
No spaces are allowed in the sketch name. Use an underscore.
You did not read the rest of the message "followed by ....." 
You need to save your file in this form: Name_Sketch "without spaces" in the name.
I think I'm right in saying that IDE v1.8 substitutes in underscores when you try to name a file with spaces, but v2.0.4 doesn't.
I know it's lazy of me to use spaces, but being more of a typist than coder, I find it quicker, and I think to lose that function is a pity.
I used underscores, still invalid sketch name.
Hi @Centabor. Please tell us the exact sketch name Arduino IDE is telling you is invalid.