Finally, I deleted my Google account in the Android's system menu and it prompted some errors. Then I reloaded the machine's firmware. Now...
Actually, I first upgraded the software from a brand new machine and then installed the Google play kit. Yes, I did try to uninstall and reinstall...
Yes, it worked for a short while... and I have been looping in this process...
I really wonder if this issue is relevant to the latest policy of Google on Chinese brands of equipment...
My Google Play Store and Youtube have not worked on my Z9S (with the latest firmware) since last week. I did try to reinstall the the play store...
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