Like Psilocubensis said, this doesn't look like a nutrient burn. At your stage 500 PPM is not that much, thats like 1.3 EC but again Autoflowers need less nutrient then normal type plants.
I tried to check out your temperature in your Diary but i don't see any info about that.
Why teperature? When looking at your picture you can see some leaves standing high meaning your plants are getting hot and goes into ''protection mode''.
What can you do?
1. Do you have a carbon filter? if so, make it run faster so more hot air can escape.
2. Do you have a kind of Air conditioning for your temperature? if so make it colder,
3. Do you have something that can control humidity? if so raise your humidity. The hotter your tent gets the higher humidity you should run.
4. If you have all or some of these things above, you can try to get a perfect climate :)
If you still have any doubts, you can and should always give your plants some water with or without enzymen and no other nutrients from time to time.
This will clean up old nutrients in your medium(soil), will keep your PPM/EC on a better level, removes salt from nutrients and more.
Good luck growing :)