Problem with update to Keil uVision 5 with STM32F4 discovery

I also have the Discovery board but only used it with Windows and the Raisonance toolchain (after a brief mess around with the Atollic IDE and embedded ST-Link). However, it doesn't sound to me like it's a compile/platform issue if everything else in the demo is working okay. You could try going back to your original setup to confirm this though.
One thing I did notice whilst using it was the USB connector is quite delicate (I have had to resolder a couple of USB mini/micro connectors on my own prototypes a couple of times after repeated use)

So my guess is from your description, it sounds like it's quite possibly a bad solder joint on the (micro) USB connector - try testing the continuity on all the pins. It will be hard to get a probe on there, so plug a cable in and probe at the A connector side (you may need to wrap some thin wire round your probe tip, or break the cable), with the other probe on either one of the STM32F4 USB pins, or a power/ground point (look in the User manual, it has a schematic)
If you have a scope, probe the USB pins to see if there is some activity on there (even if the scopes bandwidth isn't capable you should be able to see some activity)

It seems that your C compiler does not support C99 out-of-the box.

• So try to turn on C99. As for GCC, you have to add -std=c99 to the command line arguments. As for Keil try --c99. As an alternative, in Keil uVision, you can turn this on by clicking the "C99 Mode" checkbox in the "C/C++" tab in the project options (this has been pointed out by zapeitor in a comment).

• Otherwise rewrite the code: In your case the declaration of the variable within the for statement is not valid. You have to move it up; it must appear right after the function's declaration:

void f() {
  int i;

  [...]

  for (i = 0; i < 5; i++) {
  }
}

instead of

void f() {
  [...]

  for (int i = 0; i < 5; i++) {
  }
}

The latter is only possible with C99.