There are two situation and there may be two causes.

1. SC Restart but QNAP not rebooted
2. QNAP reboot and SC Start as part of reboot.

Case 1.
I have seen a 3483 port in use error when I was using SoftSqueeze. The TCP stack was not clearing data in buffers and kept 3483 connections open until a timeout which was about 3 minutes. This is a possibility in your situation. Check by restarting an SB.

Case 2.
A reboot will clear all TCP connections so in this case port 3483 seems to be unavailable possibly due to firewall or some other security setting. Check when is port 3483 opened relative to SC startup - perhaps there is a timing issue on your QNAP.