You have the answer to your problem in this thread. You also have some... well lets just say, less than spectacular answers here.
It is not nute burn. Its a salt build-up from too many nutes available in the soil. The salt build-up has caused lock-out of possibly more than one nute, Def appears to at the very least be a mag lock-out. This is because of your MG soil. Flushing is the only way to cure salt build-up. In MG soil, a flush is pointless. You would just screw the plants more than they already are. A flush WILL NOT help you, unless you get rid of that MG soil.
Best of luck to you.