Looks like I found it, it's being sent this message over and over:
Error: SiteDomain parameter not defined in your config/domain file. You must edit it for using this version of AWStats.
Setup ('/etc/awstats/awstats.conf' file, web server or permissions) may be wrong.
Check config file, permissions and AWStats documentation (in 'docs' directory).
ISPConfig 3 may have not configure this properly?