Thanks for you help.
At the end of the slapd.l.log there is the following error:
bdb_dn2entry("dc=example,dc=com")
<= key_change 0
=> key_change(ADD,a)
bdb_idl_insert_key: a [8ce7eb72]
search_candidates: base="dc=example,dc=com" (0x00000001) scope=2
<= key_change 0
=> key_change(ADD,a)
ch_malloc of 8388608 bytes failed
Assertioinsert_key:, fil7b0ch_ma
loc.c, line 57
<= key_c