Well the new 5.1.864 version of avast should be able to work with multiple ports now and not with the old port 80, you used to have to add the port as Igor mentioned.
Please check the options in Settings / Troubleshooting / Redirect settings - is it set correctly?
Effectively that would have to have port 8081 included in the list, that is what you should be checking, see image example of mine, which has port 8081 set as one of the HTTP ports.
Now unless your proxy is using a different protocol (not HTTP) possibly a VPN then I don't know if even with port 8081 listed it would work.
Interestingly your image of the webshield.ini has port 8081 listed, so I'm thinking there must be a VPN at work here. Or some other form of control/restriction.