Same issue here. I use outlook web in edge browser on a macbook air, and I've been getting logged out every night for the last few weeks. Yesterday, I was abruptly logged out during the work day while I was composing an email.
Side note: I haven't experienced the attachment issue some are describing here, but around the same time the logout issue started, outlook stopped displaying images in my email as if the links were all broken. That issue seemed to be resolved as of yesterday but we'll see if it stays that way.
Microsoft - you have many frustrated users here. Please take note and RESPOND in some way, even if just to say you're working on a known issue.