OK, just wanted to add my experience for the public record after a fair amount of hair ripping. I have a mac pro, wired apple keyboard and mouse (directly connected), running El Capitan 10.11.6. The other day I decided to turn on filevault because most things with Apple seem to work fairly well for me, so I made full backups just in case and turned it on. I used my apple id as a recovery key. It began encryption and all was cool. After the encryption, the next time i restarted, the filevault login screen came up and I could not type my password... but only certain keys failed.
Here are the keys that didn't work: i p a d h k l c m - [ ] \
Weird stuff. So I happened to have a different keyboard and I swapped that - no immediate result, but after a couple more reboots, resetting the SMC and PRAM, it started working. Not sure why, but i thought it was maybe a glitch with the initial filevault setup. I used my computer normally after that for the rest of the week. This morning, I came in and restarted my computer, same problem. And this was with the replacement (brand new wired Apple keyboard). I swapped with the old keyboard and same problem. I zapped the PRAM - same issue. Couldn't type those same characters. I reset the SMC. nope. I went into recovery and did first aid on the disk, re-chose my startup drive... nothing. shutdown / restart / remove peripherals... no effect. FINALLY, somehow, the system generated this window:
For google, here's the blurbage:
"If you were unable to use your keyboard to type your password, it may not be compatible with FileVault."
It tells you to try another keyboard or disable FileVault.
Being that it's APPLE and brand new... I can't imagine this is because of the keyboard. I googled "FileVault compatible keyboards" and found nothing. This is obviously a known issue if it has it's own error message screen. So, for now, I have turned off FileVault. That is not the level of security I was hoping for. Seems a really odd glitch and fairly easy to address I would think. Hopefully this is something that will be fixed soon. I would like to encrypt my drive.