Finally got my ASRock Rack Altra system working, after almost two months!
First, the CPU was dead; either DOA or died by not correctly connecting the 4-pin power connector.
Then, after replacing the CPU, none of my memory modules would be accepted in slots A1-D1. They would work fine in E1-H1. This turned out to be caused by the CPU cooler. See this topic.
Thanks to ASRock Rack tech support for figuring out this was the reason, even without seeing my old cooler. And thanks to other forum members for helping me out. You know who you are.