Thanks for your response! I finally managed to run memtest86 in all combinations. I have my RAM sticks in DIMM 2 and 4. Here are the results:
FAIL in pass 15, when both sticks are in.
PASS for stick #1 in both DIMMS.
FAIL for stick #2 in both DIMMS (in passes 5 and 6 respectively)
Attached are the photos for the PASSes and FAILs (expect the fail in pass 6, which I forgot to take).