Problem:
Working with a company that uses SiteGround for hosting, we setup SSH access on the website using keys. The SiteGround system forces a specific SSH passphrase for your key pair and it appears to be randomly generated, which is fine and considered fairly secure to my knowledge.
After setting up SSH config on my local machine and leaving things for a while, I came back to find that I could not login anymore. I confirmed my access should still work and that nothing has changed with the keys or passphrase but I continued to get:
Permission denied (publickey).
Solution:
New solution: try a pass phrase with limited special characters. This does require you to setup a new ssh key through the website, but has ultimately solved my issue. My original solution seemed to be that somehow logging in was associated with when the passphrase would work and when it wouldn't. This is not the case. I also talked to support but they were only able to verify that my IP address was not being blocked while debugging the issue.
Original solution: For some reason, SiteGround requires you to log back in on the their website before you can login with SSH. If you are already logged in, then you may need to log out and back in. As a guess, I think this is probably just a way they have chosen to try and combat potential bad actors, but it certainly has a lot of friction to it.
