Lenovo Yoga Tab 3 Pro (YT3-X90L) factory image, OTA update or backup needed

I was playing around trying out some new stuff with my Lenovo Yoga tab and locked it. There is an old Google account on it, but I can't remember it. I've tried so many things: 1) I've tried to factory reset it, but it's still locked. 1) I've tried to flash TWRP in the hope to do something more, but unlocking my tab in a locked state isn't allowed. 1) I was able to follow [this tutorial](https://www.youtube.com/watch?v=nCzrnnh2K1w) to minute 9:08. But the option "Email and password" is not there anymore. I guess they patched it with 6.0.1. 1) I tried to flash a stock ROM or TWRP with LenovoDownloader(QcomDLoader). The stock ROM file is: Lenovo_Yoga_Tablet_3_YT3-X90L_ENG_S200028_1609230146. Even it's a German device, this file was the only one I could find. But the LenovoDownloader won't recognize my device. The windows stay white with no entries. I guess I installed the correct driver for this device and Windows recognizes them as a removable device if I plug it in via USB. 1) I've tried to use adb and fastboot boot oem unlock. But these commands aren't working in a locked state. 1) With a trick, I could open the Settings and be full of hope to activate the developer options to set USB debugging to active. But these developer options aren't available either with this "user". 1) There is an APK for showing the "Email and password" interface. I could install it, but it force closes right after starting it. 1) I've tried several tools to bypass this FRP: WinDroid Toolkit, D-G Unlocker, Realterm, but NOTHING. What could I do further? Specs for the device: - Lenovo Yoga Tab 3 Pro (YT3-X90L) - Lenovo/YT3_l0_row_lte - 6.0.1/MMB29M/LenovoYT3-X90L_S200307_170614