I think may be the easiest way is to run strace on the obexd on the target machine?
Xperia:
write(8, "\334\215N\25\230sO\7\r\234\220=(R1\324\37j\27\3\336\246a\21I \365\350{R\253\226"..., 666) = 666
N9 is not booting anymore
See also this issue [BT] slow file transfer - #9 by deloptes