ᐅ Lawn damage after treatment – sod approximately 2 years old

Hello there ;-)

About 1.5 years ago, I installed a roll-out lawn that has established well.
At the beginning of July, I decided to aerate, overseed, and fertilize it a bit (I fertilize regularly).

I have always used a natural fertilizer from the company Schwab. However, for the last fertilizing, I used an NPK fertilizer from Eurogreen.
I aerated using the Einhell electric aerator.

Now, I have severe patchiness, partly white in color. I can't see any fungal growth.
In these areas, the grass leaves are also thinner than in the still (lush) green parts.

Here are some photos:

As of 7.7.2022

As of 20.7.2022

What happened here?
Overfertilized?
Incorrect overseeding with a less durable seed mix?
I would appreciate your assessment.

Did you apply fertilizer during the hot temperatures?

That causes the grass to burn from the inside.

Google "soil analysis" to determine the nutrient levels. Maybe some balance can still be achieved.
If it is indeed over-fertilization, this condition will likely persist for years unless the lawn is removed and the topsoil is replaced. You can try watering heavily and hope for improvement…

To me, it almost looks like fertilizer was applied without proper watering afterwards.

How much water is he currently getting?

I already water the lawn every week for 2-3 hours. After that, it turns green again. However, it doesn’t take long (about 2-3 days) with sun/heat before it looks like that again...