I think that most of the complaints come from it being a poorly made crossword. 17 and 19 are totally separate from the rest of the puzzle, most fields only have one intersecting field and many of the hints are totally vague (how anyone figured out 15 is beyond me).
17 should by all rights be "Keepers" or "Humans" because that's what every single search for the subject has brought up. One of the first search results I found said this:
"Your journey begins as you choose which faction to fight for: the peace-loving Keepers or the vengeful Cult of Storms."
And another source, apparently copied straight from the player's manual, says that the objective of the Keepers is to "Actively oppose the Cult of Storms," while the Cult of Storms' goal is to "Rid the world of both Humans and Keepers."
Even people who've played the game have no idea what else it could be. The only way the puzzle can be finished at all is by stretching the answer to the absurd. If it really is [translation of german game title], I haven't found a single resource that refers to the group as such. I've looked at dozens of websites and not once has that word been used to describe any faction in the game.
It's not "tricky" when the answer doesn't exist in any single resource.
You're right, it really is a poorly made crossword. The hint for number 15 actually refers to a single line of dialogue in the second game which isn't even on the gog catalogue. Considering this crossword is supposed to test gog knowledge, it's ridiculous that they chose that for a hint