avast doesn't block, it scans and alerts to infection but it doesn't block. Presumable you aren't getting an infection alert as you didn't mention it, so there is something else in the mix.
If there is an apparent blocking, there is some sort of conflict and this is most commonly with another security application. Remove one element of any conflict and no conflict, what that doesn't do is confirm what is at fault.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have installed ?