Openldap stops responding after a while

I'm using OpenLDAP 2.3.30-5 on Debian 4 as a users database to replace flat files. I noticed that my clients sometimes fail to query the LDAP server and report 'Cannot bind .....(-1)', I think this error means they failed to reach the LDAP server, I checked the server and I can see it is running! I had to restart the slapd again to make it work as before. This issue happens frequently and each time I have to restart slapd to make it respond to my clients, what is wrong? I'm using ldaps with a self-signed certificate.

Thank you

Amir

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