I've done bit more investigation into why this may be happening.
Zen Cart 1.3.9h
zc_install/sql/mysql_zencart.sql:
CREATE TABLE admin_activity_log (
log_id int(15) NOT NULL auto_increment,
access_date datetime NOT NULL default '0001-01-01 00:00:00',
admin_id int(11) NOT NULL default '0',
page_accessed varchar(80) NOT NULL default '',
page_parameters text,
ip_address varchar(15) NOT NULL default '',
PRIMARY KEY (log_id),
KEY idx_page_accessed_zen (page_accessed),
KEY idx_access_date_zen (access_date),
KEY idx_ip_zen (ip_address)
)
Zen Cart v1.5.0
zc_install/sql/mysql_zencart.sql:
CREATE TABLE admin_activity_log (
log_id bigint(15) NOT NULL auto_increment,
access_date datetime NOT NULL default '0001-01-01 00:00:00',
admin_id int(11) NOT NULL default '0',
page_accessed varchar(80) NOT NULL default '',
page_parameters text default '',
ip_address varchar(20) NOT NULL default '',
flagged tinyint NOT NULL default '0',
attention varchar(255) NOT NULL default '',
gzpost mediumblob NOT NULL default '',
PRIMARY KEY (log_id),
KEY idx_page_accessed_zen (page_accessed),
KEY idx_access_date_zen (access_date),
KEY idx_flagged_zen (flagged),
KEY idx_ip_zen (ip_address)
)
That doesn't help to fix it of course, but it does explain why I didn't have this problem with 1.3.9.