I am not backend developer, but my guess how to investigate this would be to add below line 195 this two lines (echo... and var_dump...). so it looks like this:
195 } catch (Exception $e) {
196 echo $this->getErrorMessageForException($e, $this->getErrorMessages($e))
197 var_dump($e);
198 $this->addFlash('error', $this->getErrorMessageForException($e, $this->getErrorMessages($e)));
And then maybe you can see what causes the problem in your case. It should show you all the information about the error variable.