It was a temporary issue, apparently G1 decided to change their API completely which required a lot of things to be changed in the back-end. 99Stack was fast to upgrade and make sure that everything worked on their end but for a while there was no way to get the password, leaving only SSH keys as login option.
They have since fixed that which support confirmed once I contacted them on the same issue. So to conclude, password authentication and SSH key authentication works great now, both on Windows and Linux servers.