no reaction after contacting support 4 weeks ago

You may have better luck with support with a support request in English. Please understand there are still language barriers in our world today. I doubt that any mother board manufacture has support staff that speaks or can read all the more common languages in the world. English is the only language that is the most universal in our world.

I cannot and do not officially speak for ASRock's support team, and I am familiar with your other thread in this forum, so I will ask you directly. What response or action by ASRock support do you want?

In your other thread, you said the +50mv default CPU Offset voltage in the BIOS was more than necessary. Is that the value with the Multiplier/Voltage Change option set to Manual? Once you change to Manual, the CPU voltage settings are your responsibility. Manual is not a default setting.

Your statement in the other thread worries me and is confusing:

"I am not willing to experiment with undervolting to get a trustworthy system".

You're using the highest TDP processor available for PCs, and using Manual CPU voltage, but not willing to adjust it? If a 50mv offset voltage is to much, isn't reducing it under-volting?

The FX-9590 is an enthusiast's processor, it must be catered to in several ways, which includes the mother board being used. VRM cooling for this processor is essential when running Prime95. Recently, Intel users have begun to discover how terribly stressful Prime95 is. Haswell processors throttle their speeds in under a minute when reaching 100C+ running some version of Prime95. Those processors have a TDP of 84W.

The manual Offset Voltage default value is probably based upon the CPU being used. ASRock has used many FX-9590 processors for testing, at least 20 or 30. You had one or two. Unless that offset voltage value is a mistake, its value was chosen to accommodate the usual range of differences in processors.

IMO, AMD made a big mistake releasing a 220W TDP processor that is "compatible" with any AM3+ board with the appropriate BIOS. That is approaching twice the TDP of the FX-8350 (125W.) Intel has guidelines/requirements for minimum VRM capability for their over clocking chipset boards. Apparently AMD does not have this.

Speaking of which, did you have AMD Application Power Management enabled in the BIOS?

Your memory problem, for memory that is not in the QVL of both AM3+ boards you used, including this one, but because it worked in the other board means it is guaranteed to work in this one?

You said you had to set the memory to 800MHz (1600) to be Prime stable, but not stable at 1866 or 2133. The QVL for this board has very few entries over 1600, since these FX processors are not known for their great memory controllers. If you can get memory to work above 1600 with these processors, you are doing very well.