Just to follow up with this--DonatoP, have you opened a case? If so, has Support helped you find a solution?
If not... I took a second look at your screen shot and when I compared your window to mine (admittedly, a managed client with client-side control, and in English ), I noticed there are two options for entering credentials: Basic or NTLM Authentication.
I'm using version 12.1.2015, the enterprise version (not Small Business). Do you have that version, and do you have this option? As you can see the latter option has no such character limitation, so you may want to confirm with your internet provider what type of authentication they require for their proxies.
sandra