What does "The file doesn't exist or can't be opened" in the rootkit-log mean?

If you get this error by clicking on “RootKit Log” in ispCP, rkhunter never ran on your system.

You have to wait up to 24 hoers for the first log (it starts every night at 00:00 o'clock via cron) or you can start it manually:

 
 # rkhunter --createlogfile --cronjob
 

After this there should be a long log entry.