Since I had the problem and have seen on the Internet that I'm not alone, I wanted to name my solution to the problem.
New PC with Z790-E + i7-13Gen + G.Skill Trident Z5 6800 + AIO water cooling built together.
Memory came in A2+B2 but Q-code 55 "Memory not Installed" always appeared.
Memory installed in B1 and/or in B2 no problem.
As soon as the memory was in A1 and/or A2 the error appeared.
I have already replaced motherboard / CPU and memory, but error persisted.
I did not follow the instructions for mounting the AIO water cooling properly.
I have not used the plastic washers for screws! (On the following picture to see which were crucial):
When I loosened the two spacer screws (which point to the RAM banks, which are marked in the picture), I noticed that there was a ladder track scratched at each screw hole.
This probably caused a short between the two tracks which caused the problem.
After I have subsequently attached the plastic washers for screws, the PC started immediately with the RAM in the slots A2 + B2 and XMP etc. could be used.
So was own stupidity with me, why this error occurred.
Maybe this article helps someone who also has this error with the A1+A2 memory banks.