Stream keeps saying that my Login has expired and I need to login again - it interrupts the vidoe

From the title it sounds like this happens as you are still actively playing the video?

We do have code that times out your session after it's idle for a while, but that shouldn't happen unless you leave the page open without doing anything for a longer time.

Can you open up a support ticket through your O365 Admin to Microsoft? The support person can help you gather needed info fiddler traces, etc to try and see what's happening.