If the former, some browsers will autocomplete to a longer URL that might include the file manager address. I've had that happen for me.
If the second or third, check the URL of the address and see of they are to the login page, or if they include other parameters. If they include a question mark and a bunch of data, try removing that. The URL should end after the course name.
Otherwise, I'm also stumped about what could be the cause.
I believe that you have hit it on the head. I use a password manager that syncs in the cloud. I went back to that password manager and in the url I can see that it will open on the File Manager page. I cleaned up the URL and it is behaving normally.
Thanks for pointing me towards culprit.