The error could simply be because you have an encrypted drive (as the message itself mentions)... Why do you want to use the offline scan? Truthfully, there are stronger scanners as reasonable options. You could download the AVG or Avira "Rescue CDs" & give them a go. My favorite is the Avast Rescue CD - if you have a friend with a known-clean PC with Avast installed, they can make one for you (it's in the "Tools" menus).
For the AVG CD or USB tool:
AVG Business PC Rescue & Repair Toolkit | Free Download
For the Avira Rescue tool:
Download Avira Rescue System | Official Website
There are many others. I like the AVG/Avira tools because they can update both the program and the definitions before starting a scan.
For an on-demand only scanner that won't conflict with your installed real-time scanner, you could try Malwarebytes AntiMalware ... just remember to decline the trial offers for the full product (which would conflict with your current real time scanner). The free Malwarebytes won't conflict, it will only run manually.
If you believe you have a present, intractable malware issue, you could try the "Security Arena" sub forum here at Sysnative.
