Hi!
Thank you for your response SisterOfMercy. Today I've read your reply and the post, and tried the following:
On Proxy Filter (Squidguard) -> Target Categories
I've created a new category called "white_list" (I didn't used "whitelist" to avoid future mistakes) and added the following domains:
msftncsi.com
131.107.255.255
also added to the regular expression box the following:
msftncsi
saved the changes, applyed the config on General Settings and tried (disconnect the cable and reconnect)
The result?
The network icon displayed a yellow triangle for an instant, but then the triangle dissapeared and now all is completely all right!
So thank you all for your help, I've added the text "[SOLVED]" at the topic's tittle so it can be helpful to others.
Greetings and thank you again! ;D