I have a Xiaomi Redmi 5 Plus with MIUI 8. My system crashed today and then the phone tried to reboot but is stuck eternally in the boot screen with the Mi and Android logo. I only had 100 MB free space, so maybe the system cannot boot because of that? I tried to clear caches with fastboot but it says "device is locked. Cannot erase". Also flashing TWRP is not possible. When trying to start adb shell it only says "error: closed". The newest version of Mi PC Suite cannot connect and an older MiPhoneAssistant version is only able to flash the ROM.
So I really tried a lot, but maybe someone has another idea with which I don't lose my data?
EDIT: So, instead of starting the bootloader I left the phone in the booting process and then I could connect with adb shell. But without root permission I don't have access to user data.