It looks like the login screen was open, but it was not being displayed properly. When the same issue occurred to me a while back I accidentally clicked enter and it logged me into Sharepoint. I'm still not sure as to why it is happening and it is still happening.
A simple workaround is to click enter as soon as this window pops-up. Make sure that you don't click anywhere else or on any other window. Directly hit enter as soon as this window pops up and wait for few seconds..
If that doesn't work, Opening Excel in Safe mode seems to work some times. But I haven't found a proper solution yet.