Well, my question is simply, "what tries Avast to do on the web when I click the register button.".
And in my opinion, avast developers/... can best assist with this question, and I think they will be better able to read English then Dutch.
Oh, this is maybe relevant squid logging:
When opening Avast:
1433602810.009 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.011 0 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.035 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.050 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.358 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.374 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602810.922 8 192.168.7.100 TCP_MISS/200 656 GET
http://www.google-analytics.com/__utm.gif? - DIRECT/192.168.7.77 text/html [Connection: Keep-Alive\r\nUser-Agent: avast! SimpleHTTP\r\nHost:
www.google-analytics.com\r\n] [HTTP/1.1 200 OK\r\nServer: nginx\r\nDate: Sat, 06 Jun 2015 15:00:10 GMT\r\nContent-Type: text/html\r\nTransfer-Encoding: chunked\r\nConnection: keep-alive\r\nX-Powered-By: PHP/5.4.39-0+deb7u2\r\n\r]
When clicking the register button:
1433602818.147 1 192.168.7.100 NONE/400 4033 NONE error:invalid-request - NONE/- text/html [] [HTTP/1.0 400 Bad Request\r\nServer: squid/3.1.20\r\nMime-Version: 1.0\r\nDate: Sat, 06 Jun 2015 15:00:18 GMT\r\nContent-Type: text/html\r\nContent-Length: 3675\r\nX-Squid-Error: ERR_INVALID_REQ 0\r\nVary: Accept-Language\r\nContent-Language: en\r\n\r]
1433602818.940 9 192.168.7.100 TCP_MISS/200 673 GET
http://www.google-analytics.com/__utm.gif? - DIRECT/192.168.7.77 text/html [Connection: Keep-Alive\r\nUser-Agent: avast! SimpleHTTP\r\nHost:
www.google-analytics.com\r\n] [HTTP/1.1 200 OK\r\nServer: nginx\r\nDate: Sat, 06 Jun 2015 15:00:18 GMT\r\nContent-Type: text/html\r\nTransfer-Encoding: chunked\r\nConnection: keep-alive\r\nX-Powered-By: PHP/5.4.39-0+deb7u2\r\n\r]
1433602818.960 5 192.168.7.100 TCP_MISS/200 657 GET
http://www.google-analytics.com/__utm.gif? - DIRECT/192.168.7.77 text/html [Connection: Keep-Alive\r\nUser-Agent: avast! SimpleHTTP\r\nHost:
www.google-analytics.com\r\n] [HTTP/1.1 200 OK\r\nServer: nginx\r\nDate: Sat, 06 Jun 2015 15:00:18 GMT\r\nContent-Type: text/html\r\nTransfer-Encoding: chunked\r\nConnection: keep-alive\r\nX-Powered-By: PHP/5.4.39-0+deb7u2\r\n\r]