Also important: disconnecting the earth in the plug of active box in room two solves the prob.
The building does not have a unified earth and there is a difference (that you can measure with your trusty multimeter as a few volts AC) between earth in room one and earth in room two.
Once you have identified this difference, tell the venue manager that he needs to get an electrician in to solve what is a serious problem.
You solve your problem by getting all your power - and therefore earth - from the one source. So now you need a nice long length of power cable.