Someone removed the Priority "Важная" from the priority list https://detmir.atlassian.net/secure/admin/ViewPriorities.jspa
How to see who did it?
How to rollback this change as for all tasks, this priority disappeared, which violated the prioritization of tasks.
You can try to look at the Audit log :
Edit: I'm not sure that this activity is logged on the audit
Try to look at the history of each issue to see if the change is saved. Try also by using the REST API
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