There are several possibilities for your problem, such as:
Did you happen to change your CPU cooler, or remount your cooler? With the memory controller in the CPU, it is possible to apply to much or uneven pressure on the CPU, and the contacts used for the connection between the memory and the CPU are not right.
Given the new case, any chance of shorts between the pins of the memory sockets under the board, to the case? Or a mother board standoff in the wrong position causing a short? Or when inserting a DIMM in a slot, pressure causing the board to bend and the same pins bending and shorting one to another?
Did you try two DIMMs, one in slot A2 and one in B2? That is the recommended placement for two DIMMs. But it does sound like the A channel is the problem.
Any chance the eight pin CPU power connector is not inserted correctly?
You are likely going to need to remove the board from the case, and try to start it on an insulated surface, the typical unmounted test setup.
You didn't fill in your system specs, which is a big help and really a requirement for posting in the forum.