Hi,
From the error message the issue seems related to the Joule Heating module that you have enabled.
How are you setting up the heat dissipation of the servers? If you already have a power source under advanced concepts, you normally would have Joule heating off.
If Joule heating needs to be on, then you have to define where the current goes in, where it goes out, and at least 1 face (not a contact face, though) with a non-zero potential. These would all be “Wall” boundary conditions.
Cheers