Global mail server settings are different than individual e-mail account settings for the user. It is separate from Roundcube configuration.
Global mail server settings are used for example for password recovery, new user creation etc.
If you received an e-mail confirmation that's good as this needs to be working as well.
Roundcube's e-mail accounts have separate SMTP and IMAP server settings.
If you can not receive e-mails than your IMAP part is not working.
First fix the ERROR: Wrong 'suhosin.session.encrypt'
If you don't have access to make necessary changes (for example this is a hosting company) you will need to contact technical support to eliminate this error.