Note that in most cases logs from at least 2 peers are required, not from one.
1. Turn on Debug Logging
Desktops and Android/iOS: Go to Sync "Preferences (Settings) > Advanced" and activate "Enable debug logging". Alternatively, on Mac and Windows, you can hold shift + right-click Sync icon in system tray and select "Enable Debug Logging".
All platforms: another way to enable debug logging is to create debug.txt file containing FFFFFFFF in the Sync storage folder.
To make sure debug logging is enabled, restart Sync.
2. Reproduce issue
Please let Sync collect logs for at least 15 minutes after the issue is reproduced to get enough information.
3. Create feedback
To open the report form go to Sync "Preferences (Settings) > Support > Contact support".
In the feedback text indicate where the log refers to -- the support ticket number the logs were requested at, additional details if there are any (for example, the role of that peer in setup - "source"/"destination", timestamp for the problem observed, the problem description in detail, etc. ). Don't forget to include in the report the names of shares/files causing problems, if any.
4. Send feedback including logs
When sending the feedback check the box "Include logs" (on mobiles press send button and choose "ADD" when prompted to send the logs along the feedback) - the logs will be sent automatically. Keep in mind that logs have certain volume and it takes time to transfer them. Don't close application/device until notified about sending done.
5. What to do if sending logs fails
If sending logs via Sync feedback is impossible of failed for any reason, the logs can be collected and sent manually. Here are the guides for desktops and NAS. For mobile peers please contact support.