The link that gets generated when sharing a post or comment is not recognized by the app.
So if I send a link to a mate and then click it myself (or he does, doesnt matter), then the browser-app opens instead of the jerboa app.
Is there a possibility of fixing this? Or is it not possible because of the different instances which generate different urls?
Jerboa does maintain a list of instances for you to select how to open in the app - list here and see my other comment on how to pick the instances you want.
The list can be updated with this generation script, it seems to get all the Lemmy instances with at least 50 users from fediverse.observer.