Hy!
I ran into the exact same issue. Cleaned chache tryed diffrent browser. Nope. So manual update is your way to go.
Files needed
9.11.0.844 Installer Build
9.11.0.898 Universal Build
Step 1.Install Forge 9.11.0.844 with the installer build
Step 2. Run it once then close it
Step 3. Rename
.minecraft/libaries/net/minecraftforge/minecraftforge/9.11.0.884
to
.minecraft/libaries/net/minecraftforge/minecraftforge/9.11.0.898
Step 4. Rename
minecraftforge-universal-1.6.4-9.11.0.898
to
minecraftforge-9.11.0.898
and place it in
.minecraft/libaries/net/minecraftforge/minecraftforge/9.11.0.898
Step 5. Delete minecraftforge-9.11.0.884 from .minecraft/libaries/net/minecraftforge/minecraftforge/9.11.0.898
Step 6. Go to /versions/1.6.4-Forge9.11.0.884 and open the json with notepad++ or wordpad
Change the line
"net.minecraftforge:minecraftforge:9.11.0.884"
to
"net.minecraftforge:minecraftforge:9.11.0.898"
Save it and run it. Worked for me
