Finally, Apache 2.2 is running fine on this server! :) Upgrading, i.e. manually merging all the changed configuration files, recompiling old Apache modules (unfortunately, Gentoo doesn’t care about this automatically) was quite an effort though. The “silent” upgrade was thus not silent at all but rather just silently ended with a no longer working Apache and the according web server downtime. Conclusion: I should really rethink whether I want to stick with Gentoo for this server or rather switch to something more administrator-friendly like Debian, SUSE or Fedora.
Make a diff!