Did you test the "Destination" in NWA before restarting the PI system?
If that didn't had any issues, instead of restarting the PI system, you can/could have tried restarting Inbound RA(Application). This will disconnect any hanging/open connections.
As the issues has already resolved now, it's difficult to find root cause.
However if it reoccurs you can try channel ping/xpi inspector to capture additional details before restarting PI system.