iMac 27" 5K 2019 – Failed to update or upgrade macOS

Hi all, this post is not a question. It's just to share my experience hoping it may be helpful for somebody

I had a lot of trouble updating macOS on a 2019 iMac 27” 5K. At first it turned out that the Fusion Drive had problems, Disk Utility could not fix – even in recovery mode. I have recreated the Fusion Drive (Terminal: diskutil resetFusion). After that, the problems were gone, but macOS could not be reinstalled. Only Internet Recovery was able to install Mojave (release version). From then on, the iMac initially ran again. In this state, new upgrade attempts failed in the same way as in the beginning. In the log there were error messages such as 

(…) Could not verify trust (5) using MobileSoftwareUpdate policy. Will try basic X509

(…)

(…) An error occurred loading the update

It turned out that a RAM module was defective. Neither under System Information -> Hardware -> Diagnostics nor in the diagnostic routine (press „D“ during the boot process) errors were reported. The iMac was upgraded with Kingston modules (lifetime warranty) at the time. But they were intact - the problem was the factory installed modules from Apple (one year warranty)! The installers of the newer macOS versions work with signature. In case of small defects in the RAM, the signing fails and the installation breaks off. My solution: Apple modules out, Kingston modules in -> Upgrade runs through.

This post from Gorilla helped me find the mistake

I used this tool as mentioned and it found the RAM errors

Regards