Didn't have the volume issues but did have the "System In Use By Another User" issue and the remotes would freeze up or just not send commands as well as one of the three RFX9600 extenders would lock up serial ports. This is what I found to be my issues:
In this system the following Philips and network devices are: (4) TSU9600, (2) TSU9400, (3) RFX9600, (1) Linksys WRT54G wireless router, (4) Linksys WPA54G wireless access points. All use static IP addresses and
ARE NOT connected to another network or the internet.

This started about 5 months ago when new firmware was loaded into one of the TSU9400 remotes. Since all the programs were built with PEP 1, I decided it would be time to re-build all the programs with PEP 2 (the most current version). Loaded all the new firmware for all the Philips devices and got to work on the new program builds. During this process I may have discovered what I think to be the problem. With PEP 2, when you convert over files from PEP 1 and don't remove the commands such as IR and RS232, PEP 2 will load and check them then add a device in your database "unknown serial device to extender o port 1, or extender 1 port 2, ect." So on one of the remotes I got "unknown serial device to extender 0 port unknown". I opened this up in my database and found there was multiple device commands on extender 0 serial port 1. Here is the extender and serial ports that the devices are assigned to:
Russound extender 0 serial port 1
CentraLite extender 0 serial port 2
Vizia RF extender 2 serial port 1
JVC RS extender 2 serial port 4
But only some of the device commands have been changed. Went and thoroughly checked each of the six programs and found other things missing or wrong in commands and scripts. During this time, the HAI OmniPro II program and the CentraLite Elegance program had also been corrupted. WTF, so fixed all of these and have not had an issue for well over a month.

So point being, not sure if the TSU9400 firmware was corrupted from download or just firmware V6.something was messed up. All current firmware 7.2.2 are installed on the remotes and 1.4 on the processors. Then, all the other equipment on the Pronto Network have been updated. So if you noticed the problem after a firmware update, I would advise you to first update all firmware and PEP 2 software, if you are using any Pronto Scripts either remove and reload them or download the newest from Philips, then look over your programs AND Pronto Scripts real close to see if anything has been moved around.