Hi Swann,
Yes, I'm running the analysis on a 64 bit system.

The matlab error message is:

Warning: Variable 'SPM' cannot be saved to a MAT-file whose version is older than 7.3.
To save this variable, use the -v7.3 switch.
Skipping...
> In spm_spm at 883
  In spm_run_fmri_est at 33
  In matlabbatch/private/cfg_run_cm at 29
  In cfg_util>local_runcj at 1510
  In cfg_util at 830
  In spm_jobman at 217
  In Labnic_1st_analysis_FACS>ana at 220
  In Labnic_1st_analysis_FACS at 86
Failed  'Model estimation'
Reference to a cleared variable SPM.
In file "/usr/local/SPM/spm8/spm_spm.m" (v4191), function "spm_spm" at line 889.
In file "/usr/local/SPM/spm8/config/spm_run_fmri_est.m" (v4022), function "spm_run_fmri_est" at line 33.

The following modules did not run:
Failed: Model estimation

Best,
Anne