"__attribute__((section(".SDRAM")))" problem

The difference between your two declerations is where in SDRAM your buffer will be placed. The decleration:

volatile int buff1_app[1280] __attribute__((section(".sdram")));

will place the buffer after the .rodata and .rwdata sections, and immediately before the heap. The decleration:

volatile int buff1_app[1280];

will place it somewhere inside the .rwdata section.

It could be that your stack and data are colliding, and the changes in memory layout is making you more sensitive to the problem. If you are using the 5.0 version of the kit, you could turn on stack checking to determine if this is the case.