I can't find this issue any where else, so i'm starting here : I have a user, was migrated months ago to Exchange online, checked his license, its OK.
He was out for a week & came back to problems displaying mailbox folder.
UPdated the outlook client on his machine & cleared sign ins. Now he get's this "a problem occurred during sign-in for firstname.lastname@example.org. To solve this problem, go to https://outlook.office365.com/owo//domain.com in your web browser. Do you want to open the webpage?" The extra slash is as found, but the names and domain changed for the innocent. I can upload a screen shot, if anyone's interested. I've tried a new machine, recreating the outlook profile, and resetting any hints of active sync history in the ECP on exchange online.