To reproduce the bug (Mac OS Big Sur 11.6.1, but also happened on Catalina):
1. Load Slack
2. Use Microsoft Remote Desktop by remoting into another machine. Make sure "Clipboard" is enabled.
After some time of using Slack and your Remote Desktop environment, they will freeze on each other. There's some race condition that causes both to hang. It seems to have something to do with the clipboard, and is often triggered when right-clicking to bring up the copy/paste menu in the Mac OS UI. I once had this freeze occur while trying to copy paste a URL out of a web browser and paste into a remote desktop session!
Slack is aware of this bug and has been gathering information on this. You may want to reach out to them.
Please fix this! It's super annoying