Sorry, I haven´t read all contributions.
But I want add my experience with this problem, assuming it es a new aspect:

TSU9600 and RFX9400; newest firmware
Windows 7 PC; 64bit OEM
Controlling of squeezeserver by TSU9600
Router: D-LINK 605
NO ENCRYPTION

This configuration was running for two days; during this time 5 respondes
"System is in use by another User"; sometimes very poor responds of the commands (10 seconds); sometimes no respond for long time (30-60 seconds).
Distance from TSU9600 to router: ca. 5m

SAME hardware/software/firmware is running absolutly troublefree (before I tested the D-Link router and now).
But: I changed the router.
WIFI is inclusive WPA encryption now; I have never seen this screen again.