I'm not sure but I think there is a regression introduced yesterday at commit 17ca9b6.

All my forms were working before I ran composer.phar update command.

After having updated my codebase, I had to update my entity type fields in all my forms to use a FQCN everywhere instead of the Doctrine entity shortcut name. In other words, before the update I had:

$builder
        ->add('foo', 'entity', array(
            'class' => 'FooBarBundle:Foo',
            'property' => 'name',
        ))
;

Then I updated Symfony and the ManagerRegistry::getManagerForClass() method modified yesterday in commit 17ca9b6 don't translate my shortcut class name to its FQCN anymore. So I'm forced to use a FQCN instead in all my forms to make it form again:

$builder
        ->add('foo', 'entity', array(
            'class' => 'Foo\Bundle\BarBundle\Entity\Foo',
            'property' => 'name',
        ))
;

So I suspect a regression with yesterday's commit.