Sophos UTM and Slack: Connection Trouble

Slack has a sense of humor, or alternatively a pretty poor sense of self-worth. I received the following error message in the browser as well as in the desktop client:

Apologies, we're having some trouble with your web socket connection.

We've seen this problem clear up with a restart of Slack, a solution which we suggest to you now only with a great regret and self-loathing.

That's quite an error message, I actually had to laugh when I read an error, which doesn't happen that often.

By the way, the Slack developers must not hate themselves, because here my Sophos UTM has problems with HTTPS scanning.

For the following URLs must be excluded from the HTTPS scans, then it also works with the web socket connection:

^https?://[A-Za-z0-9.-]*\.slack\.com/
^https?://[A-Za-z0-9.-]*\.slack-msgs\.com/
^https?://[A-Za-z0-9.-]*\.slack-files\.com/
^https?://[A-Za-z0-9.-]*\.slack-imgs\.com/
^https?://[A-Za-z0-9.-]*\.slack-edge\.com/
^https?://[A-Za-z0-9.-]*\.slack-core\.com/
^https?://[A-Za-z0-9.-]*\.slack-redir\.net/

The finished Web Protection exception then looks as follows (HTTPS scans deactivated):

Now it works perfectly. No more error messages in the browser and desktop client. I hate myself a bit for that too... :-D