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I think this might be a cryptography issue, I've seen this before, where older versions of putty don't have the necessary cryptographic extensions for the key exchange, and so it doesn't allow the connection. Check under Connection -> SSH -> Kex
Latest version of putty is available here: https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
If you already have updated putty, maybe try a different ssh client like secure CRT.
Thanks Brian and Brice, I am gonna try the same.