If you get the following error when trying to run the "statistics" utility ...
# /usr/local/psa/admin/sbin/statistics --calculate-all
Webalizer V2.01-10 (Linux 2.6.9-34.0.2.ELsmp) English
Using logfile STDIN (clf)
DNS Lookup (15): db_put fail!
... then it means that the /usr/local/psa/var/lib/webalizer/webalizer.cache or /var/lib/webalizer/dns_cache.db files were damaged or exceeded 2GB. These files are used by "webalizer" to cache DNS queries when it resolves IPs from access_log requests to Apache. Remove the file and run "statistics" again.
If you don't need to resolve IPs or statistics, you can disable webalizer by commenting the DNSCache and DNSChildren directives in the /etc/webalizer.conf and /var/www/vhosts/DOMAIN.TLD/conf/webalizer.conf files.