Hello, please advise me in problem.

Release 7.2.1_GA_2790.RHEL5_64_20120815212042 CentOS5_64 FOSS edition.

Average instantaneous load: 300 zwc clients, 100 pop3/imap clients

I use ZWC for email access and sometimes it hangs (2-3 times at day). Then I check zimbra maillog and see problem looks like this:

Code:
Jun  2 07:30:40 mailhost postfix/proxymap[19694]: warning: dict_ldap_lookup: Search error -5: Timed out
Jun  2 07:30:40 mailhost postfix/trivial-rewrite[21065]: fatal: proxy:ldap:/opt/zimbra/conf/ldap-vad.cf(0,lock|fold_fix): table lookup problem
And after that 15-30 seconds system unhangs and continue sending mail to users.

I'm not sure that ldap timeout due to the increased load on the server. And we had commercial ssl certificates on the server that are not expired.

What does this error mean ?

Thanks.