Please file bugs in the GitHub Issue Tracker. Include at least the following:
What did you expect to happen instead of what did happen, if it's not crazy obvious
What operating system, operating system version and version of Syncthing you are running
The same for other connected devices, where relevant
Screenshot if the issue concerns something visible in the GUI
Console log entries, where possible and relevant
If you're not sure whether something is relevant, erring on the side of too much information will never get you yelled at. :)
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