[Q] Can’t decrypt storage after OTA

I have a Sprint HTC One M7, which recently got an OTA update. I tried to apply this update, but now I can’t boot my phone. It was previously on stock firmware, but unlocked and rooted, and its internal storage was encrypted with a PIN. Now when I try to boot, it shows the decryption password entry screen, but my PIN doesn’t work.

Here are the steps leading up to this situation:

1. Using Fastboot, flash the stock recovery. Previously, I had TWRP 2.7.0.1 installed, but I went back to stock recovery (from this thread) so that the OTA would work.
2. Apply the update. This causes the phone to reboot and go through the update procedure. I wasn’t paying attention to the progress bar, so I don’t know if it finished or not.
3. When I next look at the phone, it’s showing the exclamation mark symbol indicating that the phone has booted into the stock recovery. Instead of entering the recovery, I force-reboot the phone by holding down the power button.
4. The phone reboots. Now I see the decryption key entry screen, which appears to have been updated in the OTA—the visual style is different from before. I enter my 6-digit PIN, the same one I was using before the OTA to decrypt the phone.
5. I press the Decrypt button, but after a second the prompt text changes to “The password you entered is incorrect.” Strangely, the number of remaining attempts does not decrement, but remains at 30. I am unable to boot my phone.

I can still reboot the phone to the bootloader menu, and I can enter Fastboot or recovery. However, the recovery is still unable to mount the data partition.

For some reason, I can take screenshots from the decryption prompt screen using Power + Volume Down.

Please help me fix this! I would very much prefer not to lose all my encrypted data, if at all possible.