QGIS 2.4 graphical modeler: error executing algorithm

I have to add more than 20 fields to a lot of shapefiles. I used to do it with the tablemanager plugin. Beside it is time consuming, misstakes can be made. So I want to automate that procedure through the graphical modeler.

I tested it with adding 2 fields and everything worked fine. But when I added all the algorithms it gave an error:

Error executing algorithm 2 [Errno 9] Bad file descriptor See log for more details

The log says:

Uncaught error while executing algorithm Traceback (most recent call last): Traceback (most recent call last): File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\core\GeoAlgorithm.py", line 217, in execute self.processAlgorithm(progress) File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\algs\qgis\AddTableField.py", line 75, in processAlgorithm self.getParameterValue(self.INPUT_LAYER)) File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\tools\dataobjects.py", line 215, in getObjectFromUri print _loadedLayers IOError: [Errno 9] Bad file descriptor

Can anyone help me understand what's happening here? Thanks!