XF 2.2 Unable to fork (FFmpeg - Windows)


We're running Windows servers and we're using FFMpeg with other applications just fine.
However, we cannot use it with Xenforo:

When entering the ffmpeg path in the back end and clicking save, we get:

ErrorException: [E_WARNING] exec(): Unable to fork ["C:\\ffmpeg\\bin\\ffmpeg.exe" -L 2>&1] in src\addons\XFMG\Ffmpeg\Runner.php at line 297

    exec() in src\addons\XFMG\Ffmpeg\Runner.php at line 297
    XFMG\Ffmpeg\Runner->run() in src\addons\XFMG\Ffmpeg\Runner.php at line 69
    XFMG\Ffmpeg\Runner->getVersionYear() in src\addons\XFMG\Validator\Ffmpeg.php at line 37
    XFMG\Validator\Ffmpeg->isValid() in src\addons\XFMG\Option\Ffmpeg.php at line 25
    call_user_func_array() in src\XF\Entity\Option.php at line 206
    XF\Entity\Option->verifyOptionValue() in src\XF\Mvc\Entity\Entity.php at line 787
    XF\Mvc\Entity\Entity->_verifyValueCustom() in src\XF\Mvc\Entity\Entity.php at line 632
    XF\Mvc\Entity\Entity->set() in src\XF\Mvc\Entity\Entity.php at line 567
    XF\Mvc\Entity\Entity->__set() in src\XF\Repository\Option.php at line 114
    XF\Repository\Option->updateOptions() in src\XF\Admin\Controller\Option.php at line 82
    XF\Admin\Controller\Option->actionUpdate() in src\XF\Mvc\Dispatcher.php at line 350
    XF\Mvc\Dispatcher->dispatchClass() in src\XF\Mvc\Dispatcher.php at line 257
    XF\Mvc\Dispatcher->dispatchFromMatch() in src\XF\Mvc\Dispatcher.php at line 113
    XF\Mvc\Dispatcher->dispatchLoop() in src\XF\Mvc\Dispatcher.php at line 55
    XF\Mvc\Dispatcher->run() in src\XF\App.php at line 2300
    XF\App->run() in src\XF.php at line 488
    XF::runApp() in admin.php at line 13

I've tried various combinations like C:\\ffmpeg\\bin\\fmpeg.exe or C:/\ffmpeg/\bin/\fmpeg.exe and others, but we always get Unable to fork.