Cannot unlock after setting PIN

Hello,
I’ve used the default Android configuration App to enabled PIN protection and to encrypt the device. After rebooting I’m presented with the PIN entry dialog (as expected) but I’m not able to activate the on-screen keypad (it’s not shown and can’t be made to appear). Touching the Pin-entry dialog, buttons displayed at the status bar or the back-button have no effect.
Luckily I’ve found a workaround: Press the emergency-call button at the bottom of the screen and then the (hardware) back button until I’m back at the Pin-entry. I now have a virtual keyboard and am able to unlock the device.
Despite the workaround this is a serious problem that should either be fixed or prevented by disabling the relevant Android functions.

Dear, could you kindly provide a photo or video to illustrate the problem and submit this bug on Feedback via Settings/ Feedback on your device at the same time for a better and faster diagnosis? Many thanks in advance!