HTTP Error 400

How are you trying to access this?  Via the web interface, or...?

Via the desktop application

It's hard to tell from exactly what you've shown, but I think this is some sort of authentication issue.  Does it work for other users?  Has it ever worked?  Have there been any changes in the vault or IdP configuration since it last worked?

Have you spoken to your reseller about helping you resolve this?

The issue is peculiar to 8users and it started from just one person. It was working perfectly as at last week Monday and no changes I'm aware of has been made in the vault or IdP configuration. 

I have escalated the issue to them but response was not forthcoming and seeing that the issue keeps increasing from one user to another. 

Your reseller is probably best-placed to start to help diagnose what's going on.  I can't explain why it would affect just some users, unless there's been some change perhaps to the users themselves inside the identity provider.