Most of the times, it helps to slow down a little during enabling hostmode. So, wait 2-3 seconds after choosing speed, then, after vbus on, wait 1-2 seconds before connecting cable (physically), then, wait 2-3 seconds, before enumerating...

Anyway, best thing You can do to help us help You is provide here "kernel messages" (dmesg) log, copied *after* unsuccessful attempt to connect keyboard.

Also, just in case, try with different speed. But, log from dmesg is most crucial here.