The latest version of Jerboa only works with Lemmy v0.18 which is still in pre-release. I'm reading rumours that v0.18 might come out today but I've yet to find any confirmation.
It's kind of weird that they would push the v0.18+ only client before releasing the ui/backend.
Edit: btw it's possible to upgrade and your account will work on v0.17.4 instances. But trying to login from a new jerboa install doesn't work