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In the small capture file I see some packet-loss and recovery. But at the time of the FIN, there is no packet-loss. There are full-size frames, but that happens at other times too. So from the capture point-of-view, it is just the sending system closing the connection for whatever reason. As you have made the trace on the receiving end, it could be worthwhile to also look at a trace on the sending side.
Normally I would expect a FIN to be generated by the application (if TCP wants to close the connection because of problems, it uses a RST). So I assume this problem to be a problem in the sending application and you would need logging and/or debugging in the application to pinpoint the source of it closing the connection. If the application development is nut under your control, I would suggest to open a case with the application developer.
