After 4 hours of searching, i got my display working.
I had the same problem with the random dots. Ive read a lot about RC circuit for reset fix...
But the wohle time i couldnt understand why the display is working with the commands and only the ram write access wont do.
My last attempt was brute force. I was very bugged and i tried the last attempt before prime time
After the brute force i saw vertical lines an i thought i damaged the OLED but after power up, my code works 100%.
for(uint8_t j=0;j<40;j++) i2c_writeReg(OLED_address, i, 0xf0);
for(uint8_t j=0;j<255;j++) i2c_writeReg(OLED_address, i, j);