Problems trying to ssh to AWS EC2 instance

I've been using an EC2 server with LAMP for a few months now and all of a sudden I can't connect to it via ssh. I run the same command on Cygwin I've been running since I've started working with it, which is:

ssh -i ./Desktop/keys/teste.pem ubuntu@54.94.211.146 -v

At first I was getting this message on the debugger:

$ ssh -i Desktop/teste.pem -v ubuntu@54.94.211.146
OpenSSH_6.7p1, OpenSSL 1.0.1j 15 Oct 2014
debug1: Connecting to 54.94.211.146 [54.94.211.146] port 22.
debug1: Connection established.
debug1: key_load_public: No such file or directory
debug1: identity file Desktop/teste.pem type -1
debug1: key_load_public: No such file or directory
debug1: identity file Desktop/teste.pem-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.7
ssh_exchange_identification: Connection closed by remote host

Then I tried reeboting the EC2 instance through the AWS dashboard. Now I get this:

$ ssh -i ./Desktop/keys/teste.pem ubuntu@54.94.211.146 -v
OpenSSH_6.7p1, OpenSSL 1.0.1j 15 Oct 2014
debug1: Connecting to 54.94.211.146 [54.94.211.146] port 22.
debug1: Connection established.
debug1: key_load_public: No such file or directory
debug1: identity file ./Desktop/keys/teste.pem type -1
debug1: key_load_public: No such file or directory
debug1: identity file ./Desktop/keys/teste.pem-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.7
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2 pat OpenSSH_6.6.1* compat 0x04000000
debug1: SSH2_MSG_KEXINIT sent
Connection closed by 54.94.211.146

I have absolutely no idea what happened. I neither remember doing anything unusual nor messing with the identity file, as the debugger suggests the problem is with the identification. I've already tried to ssh with a different machine and a backup of 'teste.pem', the errors are the same. I'm really at a loss.

Thanks in advance!