[Android Q&A, Help & Troubleshooting] openslt:
Backup of Levono Tab S8-50LC exist for Android 4.4.2 version (date 2014-10-10), but after installation you can upgrade to Android 5.0 version. Indeed, it works not only with Lenovo Tab S8-50LC, but also works with Lenovo S8-50L. I tested myself and after flashing this, I can call with Lenovo Tab S8-50L ! Files mentioned in 1-4 steps can be downloaded separately or you can download all them at once with Lenovo_TAB_S8-50LC_S139_141010.rar

1. You will need to attach tablet to computer via USB cable. Make sure your system has USB drivers for Android. If you use Linux system, please install android-tools-adb (for Ubuntu), android-tools-udev (for openSUSE) or similar package for other distribution. If you work with Windows, try look https://software.intel.com/android/articles/intel-usb-driver-for-android-devices; while for MAC you don't need additional drivers.

2. Installation of backup can be done with Intel Platform Flash Tool Lite (here you find platformflashtoollite as RPM or DEB package for Linux, also as EXE for Windows, also PKG for MAC OS). Thus at first install it. In Linux main executable is /usr/bin/platformflashtoollite . If you use Windows, make sure to install Intel® Android* USB Drivers as pre-requisities.

3. Download backup TABS8-50LC_S139_141010_WW14_BMAIN_ROW_US and extract this RAR archive. Here you will find target_bin subfolder with these files: boot.img capsule.bin droidboot.img efilinux.efi esp.img installer.cmd modem_nvm.zip modem.zip partition.tbl pft_flash_lte_normal.xml recovery.img system.img. But here pft_flash_lte_normal.xml is broken!

4. Download TABS8-50_Otkat.rar and extract archive. It contains good version of pft_flash_lte_normal.xml. Move pft_flash_lte_normal.xml and FOTA_IA32_R_0002_19_SecEnabled_From_X64.fv into target_bin subfolder.

5. Turn off Lenovo Tab S8-50

6. Open installed Platform Flash Tool Lite.

7. Press Browse button and select pft_flash_lte_normal.xml from target_bin subfolder. As configuration is correct, you should see " INFO : Ready to flash!" message in log.

8. Make sure you see Start to flash button and your tablet in Platform Flash Tool Lite. If you can not see then tablet is turned off, try boot Lenovo tablet into bootloader by pressing Volume down and Turn on buttons simultaneously in tablet until you feel vibration.

9. Press Start to flash. Procedure can take some time and you device will reboot several times – don't turn off it!

P.S. If you tried to downgrade Android or install other images before, you may see messages like Boot failed: EFI Hard drive or EFILINUX ERROR [start_boot_logic:431] No valid target found. Fallbacking to MOS in tablet. If you see these, then you need repair your BIOS. To do this, download and extract repair_Bios.rar archive. Open installed Intel Platform Flash Tool Lite, press Browse button, from newly extracted data select flash.xml to flash. To be able to flash, turn of tablet by longtime holding power button (until turn of), and then turn on by holding all three buttons (volume down, volume up and power button) until you see word Fastboot in screen. Then device will be visible in Platform Flash Tool Lite, press Start to flash. After finishing, device will stay in bootloader menu. After repairing BIOS, do 6-9 steps to flash backup!

See also https://xdaforums.com/android/help/lenovo-tab-s8-50f-stock-firmware-rom-t2989791/page135

After installing, you can step by step update/upgrade to further official Lenovo S8-50LC versions by conventional Android update tools:

to v2015-01-21 by S8-50LC_S000139_141010_ROW_TO_S8-50LC_S000200_150121_ROW_WC96.zip (alternative link) with Android 4.4.2

to v2015-06-17 by S8-50LC_S000200_150121_ROW_TO_S8-50LC_S000213_150617_ROW_WC2D.zip (alternative link) with Android 4.4.2

to v2015-10-16 by S8-50LC_S000456_151016_ROW_WCC5.zip (alternative link) with Android 5.0

to v2016-02-04 by S8-50LC_S000456_151016_ROW_TO_S8-50LC_S000478_160204_ROW_WC719C7C29.zip (alternative link) with Android 5.0