So the BIOS is updated to a newer version?
If all the possible combinations of fitment:
Stick one in slot 1 alone and then in slot 2 alone, then stick 2 in slot 1 alone then in slot 2 alone, all work, but
Stick one in slot 1 with stick 2 in slot 2 doesn't, neither does around the other way,
you're left with the need to prove whether the problem lies with the RAM, or the rest of the system.
You're pretty much left with a process of elimination, and I think the best possibility would be to get to test the RAM in another system and see if the same problem occurred. It wouldn't 100% prove the issue if another system could use it OK, but if it also failed in a second computer, that would be pretty conclusive that something has happened to make the DIMMs no longer compatible with each other.
Edited by Platypus, 10 September 2017 - 11:35 PM.