The burn marks are unusual and seem to be caused by arcing due to -
1) improper installation of memory modules
2) faulty module slot(s)
3) some other motherboard fault
4) faulty PSU
It is suspected that the motherboard (memory slots) is/are the problem here.
Could try other memory (but be prepared for possible damage again to this test memory), and/or try the original memory in another computer.
Try another known working PSU.
If problems continue, then the motherboard (slots) is/are most likely faulty.