Thanks for pic, seems to be a lot more in there than in my Corvette Next thing to start looking at / for is the circuit diagram, Warwick have these, how much detail is another matter. My best first attempt would be: unplug and replug any detachable connectors several times. Might be oxidation on contact causing poor connection (you may not have any detachable connectors - hard to see in pic). If this does not fix, next identify the treble pot connection terminals, give the wires on these these a gentle pull, if one comes off - resolder. Next follow wires back to whatever they connect into next, give them a gentle pull where they connect, if it detaches - reconnect.
FWIW one of my Warwicks (Rockbass not German one) had a badly connected wire that was not picked up in manufacture.
Are there any