Hey @Riku_Kobayashi thank you for letting us know that you’re experiencing this, we’re so sorry for the trouble.
So we can better assist you, please provide a brief description of your current setup using this link as a guide. This will be extremely helpful for future troubleshooting of this issue.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
Additionally, have you attempted a full reboot of your QNAP device and router? If not, please make sure to power off the devices and unplug them for a few minutes, plug them back in, and power them on - is there any change in behavior?
If there is no change, could you please reinstall the Roon application on your Core and restore a backup from an earlier date?
Thank you!