I'm using X-notifier in Chrome Version 65, latest version of X-notifier, OS windows 10, and it is working fine with 7 Gmail accounts, and has been reliable for more than a week. One of my accounts had allways worked, but the others failed. I gave up and removed them fom XN, about a month or so ago, I added one back, and it worked intermittently for a while, then no problems. I added another 5, and they have all been fine.
Dunno if Tobwithu fixed it, or if Google changed their process, but All is good again (for now at least).
...
I have three plain gmail accounts which have been working as far as I remember... I checked my scripts list and I'm apparently using the 'standard' embedded gmail script in X-Notifier for Chrome.