Hello. I'm getting the same error. Rather than start a new thread, I thought I'd post here also.
I installed from softaculous through cpanel. I'm using a webhost. And after researching and learning that suhosin needs to be disabled, I emailed my webhost and requested that they disable suhosin.session.encrypt.
This is the response they gave: (I'm still having the same issue, and just wondering what I should say in response to them)
"Suhosin cannot be disabled server wide. However, I think we may be able to disable it on your site with custom php.ini entry.
"I've placed a php,ini in your public_html directory and made the following entry.
[suhosin]
suhosin.session.encrypt = Off
"Peace try your installation now and see if you get better results. "
Thanks.
--Restad
edit:
I tried putting the php.ini file containing the above entry into my public_html/webmail folder (since I'm trying to install the roundcube on a subdomain, webmail.mydomain.com), and now I'm just getting a blank page... don't know if this could possibly help but just adding it just in case it can. Thanks. =)