As an avast user not an Avast Team member I can't do this. Lets see if r@vast an Avast Team member gets back to the topic and gives you an answer.
Reporting success only confirms it has been successfully sent, it will need to be analysed by one of the Avast Virus Labs Team. If it is subsequently found to be a false positive, I'm hoping that it will then allow you to Extract it (if the failure in this case was due to it being stopped again).
Also did you try the Restore and Add Exception option from the virus chest (or did that also fail) ?