Onedrive folder sharing/Microsoft Account problem

I have a folder in Onedrive that is shared with a link.  I send the link to a client who clicks on it, then enters the supplied password for this folder. So far, so good. Now the client wants to upload files to this folder. When attempting to do this, Onedrive will normally prompt the user to sign in to their Microsoft account (For edit permissions).  Onedrive no longer is prompting the user to sign in to their Microsoft account.  It keeps taking them back to the main shared folder window. When trying to sign in manually using the "sign-in" option at the top right of the page, it does the same thing. It will not prompt for the account sign in. it only takes them back to the folder window.  I have 2 clients that are having this issue. One is using a PC and the other a Mac. This never happened before yesterday.