This certificate cannot be verified up to a trusted certification
means that the issuer of the certificate is not known to Chrome. It may be a legit certificate (eg. signed by the server owner itself, or by an organization for internal use), or it might be a malicious middleman.
It might help to ask Chrome for more details and post that here - another possibilty is a real CA that fell out of favor recently. There are some cases where browsver vendors stopped to ship a certificate of a certain CA because they messed up in some way (eg. giving out certificates without any check that the applicant is a real person and owns the server)
If I click "Proceed to x..." - Is the channel still using SSL to
transfer the data
Yes - your connection to the other side is still encrypted. The problem is, what is "the other side". You have no guarantee by some publicly known CA that it is really the server you want.
Am I safe to a middleman attack ?
What other (if any) attacks am I exposed to ? ( except identity
As the server might not be the one you expect, everything you send there (or receive from there) might be not legit. It might be a "copy" of your bank site, and the attacker gets your code if you enter it. And so on...