I strongly suggest trying outside of the case. You don't see a short (unless it's really bad and catches fire). A short can just lead to weird errors.
Since your memory worked in a previous build and so did the PSU and assuming you haven't done anything stupid to damage the system (got the impression this isn't your first build) it has to be either the APU or the board. Since you've already got a second APU logic strongly suggests that it's probably the board. If you're unlucky it just killed the CPU as well.