when choosing in cb login register, I get a quick error notice

on Joomla 3.4 and cb 2.0:
when I click in cb login on register, it starts the registration page but first very quickly, I get an error notice:

Notice: Undefined index: view in /srv/www/vhosts/mysite.ch/httpdocs/components/com_content/router.php on line 67

Something is trying to link to a Joomla article as relative without an Itemid while you have SEF enabled. This could be a link on the page or a redirect maybe.

can I disable SEF without problems?

Disabling SEF should cause the notice to go away as it's caused my Joomlas content component router, which is used to reformat URLs for SEF. Ideally you should find what URL is doing that though. Do you have a login redirect specified for your CB login module to link to a Joomla article maybe?

the redirect is index.php

That wouldn't cause it then. Most likely you've a module or something on the page linking to a Joomla article that has no menu associated with it as that's what causes Joomlas content router to throw that notice. It's not specific to CB. It could be anything (e.g. a module, a URL in a Custom HTML field, footer of a template, etc..).