During installation of FosCommentBundle, we got an exception that the stack could not load form_errors block.

After a research, a "fix" could be done: you have to create a new file (form.html.twig for example) which contains exactly same data from ./vendor/symfony/symfony/src/Symfony/Bundle/TwigBundle/Resources/views/layout.html.twig

Edit your config.yml file and put:

twig:
    form:
        resources:
            - "myBundle::form.html.twig"

And FosCommentBundle works like a charm ...
After an analysis, it could be a problem link to Twig_Loader_Chain