c/startrek is the "general" community of the instance, yes.
The error message on Jerboa is a known issue - unfortunately, we're going to wait until v.0.18.1 is released, as CAPTCHA is disabled in v.0.18.0, and we're not willing to give that up at this time.
There are three main workarounds that we're aware of for this issue:
When your Jerboa login fails, simply use Android's "back" button to return to the initial screen in the app - you should actually be logged in, and able to engage with the instance, at least in a limited fashion.
There is a fork of Jerboa that's supposed to compatible with instances running v.0.17 - I haven't tested this myself, so use at your own risk.
You can access the instance through a mobile web browser - the mobile web version of Lemmy is actually pretty good, and I preferred it over Jerboa even before the version mismatch became an issue. You can even install the instance as a PWA through Chrome's "..." menu.