As I can not find any information about this issue on the interwebz, I am trying to reach out to other admins who might experience the same issue at the moment.
Since about 2 weeks we are experiencing irregular Sharepoint Online (Office 365) login issues using Internet Explorer 11, Windows 7 and an Outlook Addin called "harmon.ie". We are not sure if the Addin is involved or if it is the source of the problem, but its at least affected as it shares the login cookie with IE11.
When browsing to https://company.sharepoint.com using saved login credentials (cookie), IE11 just loops through different URIs, while the user sees a blank/white page. In the same time, harmon.ie is unable to login using its "Browser Authentication Mode". Deleting the relevant authentication cookies (forcing users to re-authenticate) or using IE11 Private Browsing mode temporary solves the problem for the rest of the day. The symptoms appear irregular, but daily for about 20% of our clients. The problem is currently not reproducible by us.
Some thoughts, but nothing proofed:
*Microsoft recently activated yammer single Sign On Authentication, which means they might have screwed up their login method
*Microsoft recently introduced some welcome Screen in OneDrive Online (my.sharepoint.com)
*harmon.ie has been updated recently to a new version which fixed "authentication issues" according to changelogs
If anyone else seems to be affected by the very same problem (cookie/authentication Office 365 issue since a few weeks) please comment below or contact me using G+ or this form: http://blog.rootdir.net/kontakt/