Gmail "Forbidden Error 403"

I have five Gmail accounts. For the past three weeks, every time I try to switch from one account to another, instead of seamlessly being taken to the requested Gmail account, I get a white screen with the words "Forbidden Error 403" written down. I have discovered that if I click on the bookmark for Gmail the requested account has, in fact, loaded. However, the 403 error message appears to be generated by this URL: https://partner.android.com/accounts/Logout2?hl=en&service=mail&ilo=1&ils=androi dpartner&ilc=2&continue=https://mail.goo…

There are many discussions of this on the Google support site, where the consensus seems to be that this is a problem related to Safari 11 updates and may have to do with the certificate. Can someone fix this please?

Here is a screenshot: