can't click inbox folder & error , can not create cache file

Dear all;
  i can login to sqwebmail now , and i can create floder ,send message, move message across different folder except click inbox folder .
when i click inbox folder , the response page is blank, and in system log file /var/log/messages, i can find these words: can not create cache file.
i need your help too.

thanks a lot 
toby 

my qmail install platform & config information:
1.qmail-1.0.3
2.qmail-ldap-20040301a.patch
3.openldap-2.0.27-8
4.redhat linux 9.0
  the base system(qmail + ldap support )is fine, i can use smtp ,pop3  and my openldap is work too.

thanks for Brendan Braybrook & Brian Candler's help, i can't fix the login problem without their help