I'm getting the same error. My connection string is 100% right. It's the same as in my old Roundcube RC1 installation.
I copied the 0.1 stable files in a new directory. Did not run any .sql update script because I didn't think it was necessary. Configured the two configuration files and saved without .dist extension. Gave full write attributes to folders: temp and logs.
Anything I missed? The old Roundcube installation is working fine with the exact same database connection string.