Internal error: throwTo: unrecognised why_blocked value

I am getting this error:

internal error: throwTo: unrecognised why_blocked value
    (GHC version 7.1.20101124 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Sadly I was unable to make a simple test case. It is a program build upon Etage with heavy FFI (X11, webcam capture, NXT interface) and threading.

Problem is reproducible. It does not happen always, but in let's say 10 % of program runs. Program is compiled with -threaded -O2 and it happens only if I pass +RTS -N2 (or bigger -N) parameter. With only -N1 it seems it does not happen.

If somebody wants I can send him/her whole program. You do not need a webcam or NXT as this happens when there is nothing connected to the computer and program prematurely exists because of this.

Or you can guide me what and how should I debug.

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mmitar@gmail.com
Operating system	Linux
Architecture	x86_64 (amd64)