there was a thread about this topic here: http://line6.com/sup...ew/thread/13813
but it is read only now, so I start this new thread to tell you about what I just found out in case anybody has a similar problem.
In my case the Volume pedal did not work. It was simply the connector at the amp-side, one of the spring contacts was a bit twisted an did not make contact.
I fixed it and now it works again :-)
If you open the pedal you can check the two outer pins (left and right) of the connector of the wah/volume-board. At these two pins you should measure about 5 Volts if connected to the Amp.