There are many reasons why your Google account might get disconnected from Instantly, for example:
Sending higher volumes of emails. In this case, there is a huge possibility that your accounts are sending too many emails because of double/triple limits, which is why Google is putting a soft lock on your accounts leading to frequent disconnections.
Turning on 2FA for the first time also could have caused a security disabling - which would be fixed by simply singing into your account on Google.
Using the same phone number for verifying multiple accounts or a burner phone numbers. We know for a fact that Google has been flagging accounts for phone number verification purpose.
Sometimes, they get disconnected due to an authorization error. The usual fix for this issue is to first log in to these Google accounts yourself, and then try to connect them.
There is no option to connect alias email accounts, since aliases use the same sending account behind the scenes, it gets exposed to your provider (such as GSuite) and they will flag your sender reputation.
To fix a disconnected account that was connected via app password, try the following:
Sign into Google in Incognito mode
make sure no other Google accounts are logged in
open this URL and continue: https://accounts.google.com/DisplayUnlockCaptcha
turn off 2-factor and then turn it back on
create a new app password and use that
After you've done all that and have a new app password, try reconnecting your account back to Instantly. In the end, refresh the page so you can get the updated status of the account.
If your account keeps disconnecting after trying the solution above, try logging in to your Google account from the web browser and reload Instantly after that.
"Can't create new access" token error
This happens when your email provider is not able to share the token with us or revoke access to the token.
This usually happens if you changed the password of your Google accounts, but did not update them in Instantly.
To fix the problem with the problematic accounts, deleting (not reconnecting) them and connecting them again is the only solution. More info here.
Another reason your account keep disconnecting is because Google is resetting access to those accounts. Were those accounts bulk-registered from the same IP or using the same phone number? If so, it is recommended to open a support ticket with Google about the app-password disconnects.
To contact Google support, click on the question mark in the upper right corner at https://admin.google.com