As far as I know, Syncthing’s local web interface is locked down by default. You should only be able to access it from localhost unless you change the settings. That said, there is some security concerns about JS in browsers making web requests to localhost, which is why I keep a password on Syncthing even though the webui isn’t remotely connectable.
A new version of the Lemmy server software was released recently, version 0.18. It appears that the Jerboa app is now expecting servers to be on that new version. It looks like you’re connecting to the lemmy.world server, which has not upgraded yet. They are waiting for a feature that won’t be ready until 0.18.1
Unfortunately, there’s not a lot you can do to fix this yourself - it’s up to the people administrating lemmy.world to upgrade their server when they’re ready.
IMO, Jerboa should not have locked out old versions so quickly.
Half-Life is also my favorite. But incompetent? He graduated from MIT!