I don't use Dansguardian, so I am not sure if you have to configure SARG for either Dansguardian or Squid. You probably don't want to configure it for both.
My guess, is that your configuration is correct now, cause you have an index that shows up and the realtime works.
If you look under:
Services - Proxy: Log rotate (this setting will conflict with SARG)
Status - SARG Reports - Schedule - Schedule Options - Action after sarg
From what I read, you should leave Squid to not rotate logs at all and have SARG do it instead.
Or you can modify the CRON job for SARG so it runs right before Squid rotates logs.
If you leave Squid rotating logs, what happens is that at midnight, it will restart and zero out the acess.log, so when SARG tries to read the access.log it will be empty, producing a blank report.
You can test your configuration by going ahead and opening up the SARG schedule and clicking Force update now. Then check Status - System Logs and it should show any errors if SARG is having an issue.
If it works, you should see updated reports.