How to debug FIFO_FULL?

Hmm, that's a tricky one. Since you have the build yourself you could look in the listing file, find the address of the ioBuffer variable, and then peek to see what's filling up the IO buffer.

var IOBUFFERADDR = 0x200xxxxx;
// check gen/platform_config.h and set IOBUFFERMASK correctly
var IOBUFFERMASK = 127;
for (var i=0;i<=IOBUFFERMASK;i++) {
  print(peek8(IOBUFFERADDR*i+5)&63);
}

Hopefully you'll see a lot of one type of number, which you could then compare with peek8(E.getAddressOf(Serial1)+3) - the actual numbers are from IOEventFlags in jsdevices.h