Perhaps this comment is useful for somebody with the same problem.
After a vanilla install of my server & hestiacp, awstats did not resolved ipaddresses into hostnames.
The file /etc/awstats/awstats.conf contained the line
DNSLookup=1
I manually added that same line to all the websites awstats configuration files in /etc/awstats (awstats.{domain}.conf) and the resolving started to work.