I spent several hours today trying to get roundcube webmail to let me log in. The eventual error was 'Invalid request. No data was saved'. That error message is less than useful, but I traced the program (index.php) and found it was not setting the cookies properly. Had the CGI server dump out what it thought was going on, and spotted that the cookie was getting set, and promptly deleted. Anyway, if you're trying to get this working via CGI (php7, debian), grab the latest version off github (current commit is afa03008c7da4162e1705f0e27179802001f589d), it just works with no issues.