Authentication Fail Using Outlook Client/Gmail/POP3/VPN

Does this issue occur recently? If so, have you done any operation before that? Such as changing the password?

fails when Google/Gmail tries to authenticate it

Is there any error reported? If possible, please provide screenshots of relevant problems. (In order to avoid the disclosure of your privacy, please remember to hide your personal information)

According to your description, I notice that the issue is related to the primary account, please try to create a new profile(control panel>mail>show profile>add) and add the primary account to check if there are some differences.

In addition, in order to further confirm if the issue is related to the account, it's suggested that you could log in to web mail to check if it can run normally.