As outlied in the post you were linking to, you have to edit the settings for each category first to allow subscriptions in the forums in these categories.
Looking for german Snitz forum? Suchst Du ein deutsches Snitz Forum? Hier findest Du eins...
So you didn't set the allowed subscription level in admin --> feature configuration as outlined in the instructions. You need to allow at least 'per forum'
Looking for german Snitz forum? Suchst Du ein deutsches Snitz Forum? Hier findest Du eins...
Now the subscription level in admin --> feature configuration is set to ""Subscribe by Forum" and I still get the same screen as the one I showed a few messages away.
I am perplexed... do you think a fresh install will do the trick?