If an (guest-)user comes to an URL and then clicks the cookies bar he always is redirected to the start page. This is annoying because many users come from Google to forum threads, for example, and are thus redirected away from there.

Latest Patch is installed, redis Cache is enabled, 'guestTermsBar' template isn't modified.

I would advise testing this on an unmodified theme and third-party applications/plugins disabled. Unfortunately, I am not able to reproduce this on the latest release on a base test instance.

Yes, this was a bug they fixed in 4.7.12. The fix for it was in the globalTemplate template, which is one of the most edited.

You're missing that change most likely.