For one of my customers we were facing a strange issue - sign-in in Office 365 desktop applications (Word, Excel, PowerPoint) was not working, it looked like this:
When clicking Sign-in, entering user name and password, it told that Your request can't be completed right now.
We were using federated authentications, there were no errors in ADFS logs. A quick Google-ing suggested that Azure AD Conditional access could be blocking the sign-in, but in Azure AD there were now Conditional Access policies enabled.
After some struggling, I found out that there are Classic conditional access policies and indeed once disabling them the issue was solved. Be sure to check these also!