Losing SSH access to Acquia Cloud: "permission denied (publickey)"
I haven't determined the root cause of this issue, but I found a functional solution: 

When I get this issue, rather than in my IDE I ssh to AC via the Acquia Dev Desktop interface (clicking the small terminal icon next to the SSH address). This runs a specific SSH connection with several options:

ssh -p 22 -i "/Users/username/.ssh/acquia_key_name" -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null sitename.dev@server.devcloud.hosting.acquia.com

After this command is run (in my case in Mac Terminal), I can access the git repo again via my IDE.