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 yoga tab and locked it. 

I've tried to factory reset it, but it's still locked.
I've tried to flash twrp in hope to do something more, but to unlock my tab in locked state isn't allowed.

There is an old Google Account on it, but i can't remember it and now i'm sitting in front of my tab and staring on it and thinking about how to help my self. I can't, i need your help :(

thank you in advance for any help i can get 

UPDATE:

I've tried so much things now.
1.) https://www.youtube.com/watch?v=nCzrnnh2K1w
I was able to follow this tutorial to minute 9:08. But the option "Email and password" is not there anymore. I guess they patched it with 6.0.1.
2.) 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 its a german device this file was the only one i could find.
But the LenovoDownloader won't recognize my device. The windows stays white, no entries. I guess i installed the correct driver for this device and Windows recognizes them as removable device if i plug it in via usb. 
3.) i've tried to use adb and fastboot boot oem unlock. But this commands aren't working in locked state.
4.) With a trick i could open the Settings and was full of hope to active the developer options to set usb debbuging to active. But this developer options aren't available either with this "user".
6.) there is an apk for showing the "Email and password" interface. I could install it, but it force closes right after starting it.
7.)I've tried several tools to bypass this FRP: WinDroid Toolkit, D-G Unlocker, Realterm, but NOTHING.

What could i do further?
I'm out of ideas right now

Specs for the device:
Lenovo Yoga Tab 3 Pro
YT3-X90L
Lenovo/YT3_l0_row_lte
6.0.1/MMB29M/LenovoYT3-X90L_S200307_170614
I've wrote it down. Translation by google, but i think it's all good :) Sry for not posting, needed some time.

Situation:
"Confirm account: The device has been reset. To continue, sign in with a Google Account that was previously synced on this device. "

Explanation:
The device has been blocked by repeated input of the wrong password, pattern, etc. and can only be unlocked with the Google account of the owner.
A factory reset or hard reset does not help here, as the account information is saved in its own FRP partition and will not be deleted when the factory state is restored.

Solution:
1. Connect device to a Wi-Fi and Internet access.
2. Before confirming "Next", select another hotspot here
3. Go to "Advanced Settings" and change the proxy settings of any hotspot to Manual.
4. In the text input field of the proxy settings you enter now any word: e.g. Test
5. Now mark this word and select "Assist" in the context menu.
6. Selecting this "Assist" option opens the Google search application.
7. You can now not only browse the Internet through this search, but also search for and open installed programs.
8. We are looking for Chrome, open the internet browser, go to www.google.com and search for DPC test.
Link: https://www.apkmirror.com/apk/sample-developer/test-dpc/test-dpc-4-0-6-release/test-dpc-4-0-6-android-apk-download/
9. After downloading this application, go back to the Google search application.
10. Here we enter "Download" to search for the download application.
11. In Downloads we can now find and install the downloaded app.
12. To allow the installation of this .apk, you must allow the installation of "unknown sources" in the settings.
13. Either you go here on the installation itself on "Settings", or you search for "Settings" on the Google search app.
14. After completing the installation of the DPC-Test program, start it with "Open".
15. The app itself is will register as a device owner and now allows us to set up a new Google Account as owner.
16. If this is set up, it will also be included in the FRP partition.
17. In the event of a new factory reset, the device is reset, the DPC-Test program is removed and the device can be freshly set up in the factory settings.

after the takeover of the owner, the reason for blocking the device was not present anymore and the old owner was overwritten. This will be overwritten in the FRP partition aswell with the new account and the old one no longer queried.