My users use the start page portal.microsoft.com. I thought there was no difference between portal.microsoft.com and office.com is portal.microsoft.com only for admins? what is the best way to log in to the browser o365 fastest? office.com/login ?
Yes, there is a difference, Office.com is the default start page for users, this was mentioned recently in a message center post:
"Office.com has evolved to pull a user's most relevant apps, documents and places where they are working —all in one place--and consequently will be the default page a user lands on when signing into Office 365 at Office.com"
portal.microsoft.com will take you to the admin center, which users won't have rights for.
So yes, get users to open office.com instead and see if that works as expected. Hope that helps.