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Where to find it: https://confluence.atlassian.com/adminjiraserver/logging-and-profiling-938847671.html
If you don't have direct access to server, as Jira Administrator, you can use build in support tool to create support .zip file. You can find it in Troubleshooting and support tools:
From Confluence we need: atlassian-confluence.log
Where to find it: https://confluence.atlassian.com/doc/working-with-confluence-logs-108364721.html In
| Warning |
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In some cases, we will ask you to enable additional logging levels for our application, if attached logs, do not provide sufficient information. |