There is a blue box in the bottom right of my screen

My administrator disabled task manager, so that will not work.

Looks like that Purple box is getting in the way. An Add-on or background service might be running within Teams that isn't displaying correctly. 

Could be a bunch of things;

1. Check for a Teams update? or get your admin to repair the install?

ii. You may be able to close the "purple box" if it's shown when you press Windows+Tab (or Alt+Tab)

C. From the system tray (behind the purple box) you can right-click on the Teams icon and Quit. Then restart the application.

If it's a display issue, you could try;

  Checking your system display settings (right click on desktop), if scale is 100% and it's still there, then

  Toggle Disable GPU acceleration on/off within Teams settings

I have the same issue before (blue box at the bottom screen) but it resolved :) . task manager > application > Microsoft Teams Notification > end task > blue box disappear. Thanks to Ru for giving me the idea.(sorry for my english)

I keep on seeing the same blue box. It started showing once I started using a 4K external display that is configured at 150% scaling.

The one way I found to get rid of it is use Task View (hit WIN-TAB) and than an X appears next to the blank blue box - click it to close the box.