11.0-CURRENT #1 r267422: OpenLDAP fails to startup out of the blue after buildworld

After updating ports yesterday with all this ICU update horror today
slapd rejects to start out of the blue after months of working like a
charm:

[...]
5399feba slapd startup: initiated.
5399feba backend_startup_one: starting "cn=config"
5399feba config_back_db_open
5399feba send_ldap_result: conn=-1 op=0 p=0
5399feba backend_startup_one: starting "dc=dumami"
5399feba mdb_db_open: database "dc=dumami":
dbenv_open(/var/db/openldap-data/). 5399feba mdb_db_open: database
"dc=dumami" cannot be opened, err 13. Restore from
backup! 5399feba backend_startup_one (type=mdb,
suffix="dc=dumami"): bi_db_open failed! (13) 5399feba
slapd shutdown: initiated 5399feba slapd destroy: freeing system
resources. 5399feba syncinfo_free: rid=001 5399feba syncinfo_free:
rid=003 5399feba slapd stopped.
/usr/local/etc/rc.d/slapd: WARNING: failed to start slapd

According to that useless "suggestion" to restore from backup, I
restored the configuration and the users from backups. slapadd works
fine. But then starting the server fails again.

Via "portmaster -f openldap24-server" I tried to rebuild all ports
necessary for that fragile OpenLDAP thing, but still no success. I can
not find any hints in the log (using -d1 or -d257 starting slapd)
except the failure shown above. Since the very same configuration and
dataset worked for months now and even after the massive icu-related
update of ports yesterday (ended by restarting slapd), I wouldn't
expect any usefull hint.

Can anybody offer suggestions, please? I'm out of ideas. I find it very
strange.

Regards,
Oliver