Roundcube gives a white page

Roundcube calls back to your control panel website to determine the correct IMAP and SMTP settings for a given mailbox.

Your control panel URL must be accessible from the PHP container running the Roundcube website and located on your control panel server. Your control panel must have a valid SSL certificate (either auto Let's Encrypt or your own custom SSL). If either of these are not true then Roundcube will give a white page on login.