Connection is broken and when re-entering credentials we see:
This is the same day an update was released:
As per marketplace version listing at https://marketplace.atlassian.com/apps/1220099/jira-server-for-slack-official/version-history
Version 2.0.20 was mistakenly released with an option that forces a specific HTTP protocol when calling Slack that effectively breaks communications. This version removes such option.
Updating of an add-on should solve your problem.
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