The default config has the following line: checkpoint 32 30 # <kbyte> <min> However, OpenLDAP doesn't like comments if they're not the only thing on a line. This causes the daemon to abort startup with no explanation to syslog. If slapd is started with "-d 11000", it spits out a tiny line among all the other debug noise that says /etc/openldap/slapd.conf: line NN: <checkpoint> extra cruft after <kbyte> <min>. Moving the comment to another line solves the problem.
Looks specific to 2.4.x. It was never a problem in 2.3.x. Fixed in CVS.