OK, using the 15.08 SDK preview everything worked fine now. No errors.
I'm glad it helped!

For debugging reasons, is there a unminified version of the b4w javascript library or do I have to unminify it myself every time I change the SDK version?
You can develop an application within the SDK, which means that you'll have an access to the dev version(source code) of the b4w engine (source code is placed here: <path_to_sdk>/src/*).

Unfortunately, this is a bit unstable at the moment even in the Developer Preview (we are currently working on this feature). So you need to wait stable release.
The developing process would also be described here in the documentation:
https://www.blend4web.com/doc/en/developers.html#sdk (it's a bit outdated by now)