Ok, I uninstalled gimp 2.8.4 and erased .gimp2.8 user folder, starting from fresh I reinstalled and placed the g'mic files to installation folder, no success. Then moved those files to user folder to test, no success.
ahaha MYSTERY SOLVED ! AT LAST !!
I've re-launch gimp with
Comodo behavior blocker disabled BAM! It works !
Now I just remember having problems with some exotic executables that were silently blocked or sandboxed by comodo before. I guess behavior blocker considers g'mic as suspicious because of it's online update feature.
As disabling behavior blocker globally is not an option for me, I'm now trying to give g'mic a trusted status, and that's not easy, comodo is damn serious about security (especially with no mainstream stuffs). Ok, now it's done, I had to move g'mic exe from unrecognized to trusted files, to define a behavior blocker exception for the whole gimp folder and to define it as allowed application in HIPS rules. Phew...
Note:
I submitted gimp_g'mic.exe to comodo database for file analysis, hoping to prevent future troubles among comodo users.