Hey there 
at first sorry for my bad English!
I had the following issue in the past:
I wanted to transfer a WhatsApp backup to my new deGoogled, there are 2-3 ways to do that.
- transfer the data to the new device by copying it to the new (I think it is stored in the Android Folder of the Device /disk/Android/media/com.WhatsApp , but I am not sure!)
- transfer the data via WIFI diereckt with WhatsApp it self
- Backup the Data to Google and when you instal WhatsApp to the new phone WhatsApp searches automaticly for it.
For any reason 1 and 2 didn’t work at all in my case.
So I had to take the 3. path. I don’t like it, because its Google, but I needed my Backup…
Disclaimer: The Problem is just there, if you already signed in to google on this device.
To the issue:
- I backuped my data on the old phone
- I installed WhatsApp
- I signed in
- than where there a side, were I could select, if I want to restore a backup or not and I selected “Yes”
- then was Pop-up where I could select which Google account I want to use for the back up. With the following possible answers:
- an empty Button
- “signup for another account”
here is the problem.
If you press the empty button, nothing happend
If you press “signup for another account” you will be rerouted to google, but if you try to Signup Google says to you that you already locked in.
I had to try some time to find a workaround:
Go to Settings => Accounts => Google
Remove the google account sign in from your accounts
Now try WhatsApp again maybe you have to reinstall WhatsApp.
I hope I could help you!
P.s.: I didn’t made an bugreport now, maybe somebody want to do that or tells me where I can do that.
P.s.: I know that WhatsApp is terrible for privacy, but I need it for work and family, If you can please switch to better platforms like Signal or even better: Matrix.