And did you change your settings? Show us config show horde without the DBPassword entry and config show imp.
[/Apr 19 23:40:02 sme-server2 php: PHP Parse error: parse error, unexpected T_LNUMBER in /home/httpd/html/horde/config/conf.php on line 42
The above is an issue, and would explain why you get a 404 with webmail. On a stock install, this shouldn't happen. What is on line 42? On my test box, I have this - // 125Authentication. Have you any custom-templates created for horde, that you could remove. If you sill don't see anything post your horde/config/conf.php file from the // 110SQL to // 130Signup, again remove the password section.
John