The server hasn't been updated yet to version 0.18.0 which is required for the version 0.35 jerboa app to work with it.
0.18.0 doesn't have captcha and apparently has some problems when upgrading, so we're waiting on 0.18.1. (Shouldn't take longer than a few days.)
There was a post from the Admins earlier today describing the situation in full. Jerboa still works, just hit okay. The instance will be update in a few days when the Lemmy update is actually ready.