As Avast has developed, it has grown new features which have made it ever more sensitive to the presence of other security software installed alongside Avast. I would consider disabling the new Avast HIPS (found in File Shield -> Sensitivity). Other firewalls possess HIPS in some form or other. Outpost Firewall 2009 free is a very 'manual' one (and I do set Firewall Policy to Allow Most) and this may be why Avast 2015 R3 gets along with it so well. I do wish that the Avast Emergency Updates would use fixed name .exe files because I could then use Avast with Outpost Firewall Pro 9.1 (this is also Agnitum's fault for not allowing executables in nominated folders to be ignored).
Avast 2015 (2015.10.2.2018) refused to run with Windows XP SP3 regardless of what was installed so AVAST 2015 R3 SP1 (2015.10.3.2225) is a major improvement for me, HTTPS scanning, HIPS and all.