How exactly can I unlock Oisten Jägerhorn?

Solution 1:

You have to press the "follow" button at the Killing Floor 2 Community Page

I'd suggest to do this while the game is closed, if that didn't work unfollow and try it again with the game fully launched (albeit it should work with the game closed). If that also didn't work, go to your inventory in the game (while still following the community page), which will refresh all your items every time you go there and then go to gear. Oisten should then appear between all your other playable characters.

And if that didn't work, I'd suggest to write a bug report to Tripwire.