ldd /usr/local/libexec/slapd
linux-vdso.so.1 => (0x00007fff1f7ff000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00007f8095e98000)
libsasl2.so.2 => /usr/lib/libsasl2.so.2 (0x00007f8095c7e000)
libdl.so.2 => /lib/libdl.so.2 (0x00007f8095a7a000)
libcrypt.so.1 => /lib/libcrypt.so.1 (0x00007f8095843000)
libresolv.so.2 => /lib/libresolv.so.2 (0x00007f809562c000)
libwrap.so.0 => /lib/libwrap.so.0 (0x00007f8095423000)
libc.so.6 => /lib/libc.so.6 (0x00007f80950c2000)
/lib64/ld-linux-x86-64.so.2 (0x00007f8096428000)
libnsl.so.1 => /lib/libnsl.so.1 (0x00007f8094ea9000)
I'm not sure whether the problem in certificates could be the reason for this.
In my slapd.conf I've following lines related to TLS.
TLSCACertificateFile /usr/local/etc/openldap/ca-bundle.crt
TLSCertificateFile /usr/local/etc/openldap/slapd.pem
TLSCertificateKeyFile /usr/local/etc/openldap/slapd.pem