Please help me if you can
Since about 10pm local mountain time last night, I haven’t been able to sign into my account which is through Pokemon Trainer Club. I instantly receive the “Unable to authenticate. Please try again” message every time like it’s not even connecting. When I click the forgot password/username links, the page it goes to looks strange too. I’ve literally done every step I can find and nothing has helped. I’m hoping there is someone out there who has experienced similar or has other ideas. Please help!! I don’t know what else I can do...

Answers
I'm not sure if it is the same, as I was signing on with Google; but I just had a similar problem on my S7. Here is how I fixed it:
1) Uninstall PoGo
2) Clear Cached data for all apps. I tried to figure out which app was causing the problem to no avail, so I cleared them all. This option is under "storage" (not "apps").
3) Resolved security issues with my Google account (may not apply to you)
4) Deleted any battle parties on my other device.
5) Reinstalled PoGo
I'm not sure if 3 and 4 are necessary; but I was tired of messing around ?