pylon:
I've got some simple server code that uses the String class and had the server respond to a client request 500,000+ times with out a crash. Perhaps your code is defective.
If you are very cautious and know where the class is defective for en embedded environment, this is possible. But generally it's probably the worst decision of the Arduino makers to have this class included in the IDE. The way the OP uses the class he won't be able to answer 500'000 requests. And he does nothing the documentation says you shouldn't do.
Well, guess we will see if the deletion of the String class fixes the OP's issue. Specifically which String useage in the OP's origional posted code is causing his issue? Did the String memory location "go out of scope" in the OP's code? The issue you pointed to (below) may well fix any "bug" issue by emptying the memory location such that there is no longer memory that needs to be freeded in that location. I read closely the anti String posting and don't remember ever actually seeing an issue being fixed by the removal of String useage. I'll observe to see how this plays out.
String buffer = "";