The team at Slack are investigating an issue with custom apps not sending messages and some trouble with Slack Connect:

If you are experiencing the separate symptom relating to Slack Connect where some users are having issues @-mentioning channel members, please reset your cache and refresh by following the below steps:

1) Open the Slack desktop app, then click Help in your computer's top menu bar. (If you’re on Windows, click the ☰ three lines icon in the top left of the Slack app first.)
2) Select Troubleshooting.
3) Click Clear cache and restart.

You can find the latest updates on these issues on the Slack status site.