Contao session shared across applications

I'm developing multiple sites on my local host using Symfony WebServer Bundle (so, the browser probably sees them as one, because the host is always 127.0.0.1:8000).
Now, I'm getting the session variables populated from another Contao installation in a completely new Contao project. In other words, it's the same session data across multiple local Contao installations. The data refer to extensions which are not installed, non-existing database columns etc. which leads to fatal errors of course.

How do I clear this session? What I tried so far:

- cache:clear (both prod and dev)
- manually deleting prod and dev cache folders
- clearing browser's cache
- manually clear browser's cookies
- manually deleting PHP session files from the directory in "session.save_path" ini setting
- restarting browser
- rebooting

What else do I miss here? Please help.