1. If you see a white page there MUST be some error in log (Roundcube log, http server log or system log) or your setup is broken.
2. If you see Mail interface after login, then there's something wrong with ajax requests. Check browser console again, check Network tab.
3. There's no email preview in folder manager, what the f*** are you talking about? Do you see folder details in the right frame? If not, maybe you have iframe protection issue. Google it.
4. Could you use a different imap client (Thunderbird) and subscribe INBOX. Why it's unsubscribed? It's not normal, investigate that.