[TYPO3-mvc] Error handling for db requests?

[Christian Essl]

The error handling of db requests sometimes drives me crazy...

As an example I simply want to get all records from a table: (Strangely, I have _exactly_ the same code working inside another action)
$this->themenschwerpunktRepository = t3lib_div::makeInstance('Tx_Extensionname_Domain_Repository_ThemenschwerpunktRepository');
$this->view->assign('themenschwerpunkte', $this->themenschwerpunktRepository->findAll());

No values are assigned. No I want to debug the output and try the following:
var_dump($this->themenschwerpunktRepository->findAll());
or
<f:debug>{themenschwerpunkte}</f:debug>

Both ways lead to a blank page without any errors. (Status 200 OK).
Error reporting etc. is all configured in the Install Tool. But these settings do not seem to work for extbases persistence layer.

Here my two Actions:
http://pastebin.com/nRQ7uZYJ

I can't figure out, why the same code seems to work fine in one action, but leads to no found objects in the other actions. (and is not debuggable)

Greetings,
Christian