psteen:
Ok, I got a new oled display of exactly the same type.
Unfortunately the result is the same, no difference.
So, now what ?
UPDATE!
I changed back the constructor to
U8X8_SSD1306_128X64_NONAME_HW u8x8(/* reset=*/ U8X8_PIN_NONE);
And recompiled my application, now it works as it should!
I tried to change back to the oled display that I suspected to be broken, and now confirms it is broken ot works differently than the other exactly same working device!
Device broken!
Thank you all very much for your effort supporting me with this issue.