I have got the same trouble with Gimp 2.8.0. I am French and there's the letter 'é' in my Windows username. I tried to create another user and this time, it was ok.
Sorry for my poor English!

We need somebody with a debugger to tell us where exactly the error happens,
otherwise we can do nothing.

GEGL is not using 'g_open' and friends from <glib/gstdio.h>

in GEGL sources root

grep '[^a-zA-Z0-9_]g_open *(' gegl/{*/,}*.c

but
grep '[^a-zA-Z0-9_]open *(' gegl/{*/,}*.c

gegl/buffer/gegl-buffer-load.c:  info->i = open (info->path, O_RDONLY);
gegl/buffer/gegl-buffer-save.c:  info->o    = open (info->path, O_RDWR|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH);
gegl/buffer/gegl-buffer-save.c:  info->o    = open (info->path, O_RDWR|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR);
gegl/buffer/gegl-tile-backend-file.c:      self->o = open (self->path, O_RDWR|O_CREAT, 0770);
gegl/buffer/gegl-tile-backend-file.c:          self->o = open (self->path, O_RDONLY, 0770);
gegl/buffer/gegl-tile-backend-file.c:      self->o = open (self->path, O_RDWR|O_CREAT, 0770);

I'm not an expert, but I think there are combinations of utf8 path
names, system code pages etc. for which using libc 'open' fails on
Windows.

I don't know whether that code is executed or not with cage tool or
View->Use GEGL, but it will likely be when GeglBuffer will replace
TileManager, and since the swapdir is under user_cache_dir...

Confirming: after creating a User 'généviève', running GIMP from a DOS prompt

"%ProgramFiles%\GIMP 2\bin\gimp-2.8" --verbose 2>out.txt

selecting View->Use GEGL or using the cage tool makes crash GIMP on an
assertion failure in GEGL:


(gimp-2.8:568): GEGL-gegl-tile-backend-file.c-WARNING **: gegl_tile_backend_file_ensure_exist: Could not open 'C:\Users\génèviève\AppData\Local\Microsoft\Windows\Temporary Internet Files\gegl-0.2\swap\568-3': No such file or directory

(gimp-2.8:568): GEGL-gegl-tile-backend-file.c-WARNING **: unable to seek in buffer
**
GEGL-gegl-tile-backend-file.c:ERROR:gegl-tile-backend-file.c:957:gegl_tile_backend_file_ensure_exist: assertion failed: (self->i != -1)

(script-fu.exe:3888): LibGimpBase-WARNING **: script-fu.exe: gimp_wire_read(): error


After changing 'open' to 'g_open' (and 'access' to 'g_access') and 
recompiling, repeating the same steps does not crash GIMP

Fixed in GEGL master.

commit 48c1d6ac8925c8b015748c2a445268ddad842d3b
Author: Massimo Valentini <mvalentini@src.gnome.org>
Date:   Fri May 18 15:02:19 2012 +0200

    gegl/buffer: use g_open,g_access instead of open,access
    
    Bug 675591: Errors due to non-Latin letters in the name
    of the user profile.

*** Bug 698688 has been marked as a duplicate of this bug. ***

One thing that could be useful for other people experiencing this bug, while the fixed GIMP & GEGL versions are not yet released, is that a workaround exists.
The workaround is to type the following three lines in a DOS window for starting GIMP (the first line is not compulsory):

cd %USERPROFILE%
SET GEGL_SWAP=RAM
"%ProgramFiles%\GIMP 2\bin\gimp-2.8.exe"

I have put this in a .bat file which I now use for launching GIMP.

This is the solution for the problem, thanks!!!

*** Bug 707279 has been marked as a duplicate of this bug. ***

Thank you very much indeed, Massimo,  for your diagnosis. Since the workaround in question seems to be  mostly in Chinese for me, I've done what billy_kid did  3 years ago. A new user account has been created with no silly Hungarian characters like é, ö, ü, etc., and it has worked. God save nations using non ASCII characters.

*** Bug 751175 has been marked as a duplicate of this bug. ***