As long as you only alter the table stucture in the database, not delete anything then i dont see why you would loose any information. Sent messages are stored on your IMAP server. You said you have a backup any way so you can always restore the DB if you need to.
are you using firefox? a quick search found this The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression.. could be worth a try. if you are using ff does the message also occur in other browsers?
As mentioned above, IE6 gave me a blank screen, FF gave me the error.But, the problem has been cured by my hosting support man.The server PHP required a change, Quote from him:"The changes were made in the main php configuration file on the server after examining the contents of the script error log."End-Quote:exactly what changes were made to cure my problem are unknwon. All I know is there was nothing wrong with my install and it now works in whichever browser I use.
[root@mail logs]# rpm -qa phpphp-5.2.8-1.el5.remi[root@mail logs]# pwd/var/www/roundcubemail-0.2-stable/logs[root@mail logs]#