Ender 3 large flat surface printing problem

Hello,
i am printing large flat surface (plate): 210cm(W) x 201cm (L) x 2,5mm (H). Some technical characteristics:
Printer - Ender3;
slicer - Cura 4.4.1;
Filament - PETG black Devil Design;
built plate temperature: 75 degrees C;
nozzle temperature: 235 degrees C;
layer height - 0,2mm;
retraction - 6,5mm;
retraction speed - 25mm/s;
print speed - 50mm/sec;
combing mode - all;
fan speed at layer 1 - 0%;
fan sped at all other layers - 50%.

When printing first 2 layers, they were perfect: surface of the printing part is very even, no stringing. But at the 3rd layer strange thing happened: layer became uneven. It seems like the veins popped up from the surface. Also stringing occurred.
Attachment 14056
I have no clue what causes this problem, because layer below corrupted layer is completely flat. I have tried to increase/decrease nozzle temperature by 5 degrees C, increase/decrease fan speed but these changes do not had any effect. Maybe someone have faced with similar problem?

Can not see that picture, it is too small. First thing, no fan at all, I print PETG all day with the parts fan removed from the machine. Second you are using an Ender 3 so could very well be a mechanical issue which I can not help you with.
Sorry I not much help but I other than the fan I see nothing in your setting that would cause issue, assuming a 6.5mm retraction is correct for the Ender
PETG does not like thin layers in my experience so if you can increase your layer height it may be helpful.