This is the story as a translator /a/non said WAY back when chapter 1 of dungeon seeker manga was translated here.
Basically Arifureta was a little known Japanese webnovel, and the author of Dungeon Seeker was a reader, apparently he thought something like "I can do better" so he rewrote Arifureta in a way he thought would be better. No big deal, that's how Dungeon Seeker starts. Of course he was right. Dungeon Seeker is a more interesting story early on, and it's moderately better written (that isn't hard to do btw, it's not like Arifureta was any good), anyway as the webnovel went on the author of dungeon seaker got lazier and lazier, and started to pull parts of chapters of Arifureta changing names only. No one noticed because at the time Arifureta was basically unknown. anyway the whole second half of the webnovel of Dungeon seeker is apparently just Arifureta with different names (yeah he kept the worst part of it) because apparently daily webnovel content was harder to do then the author of dungeon seeker thought when he started.
ANYWAY, this is where the story gets slimey, apparently around the time Dungeon Seeker got a light novel (arifureta was still a moderately unknown webnovel) a fan of arifureta picked up the dungeon seeker LN, and saw some similarities, so he went looking for the webnovel, only to realize the whole second half was plagiarized. So he contacted the author of Arifureta, who then sued everyone involved (apparently the dungeon seeker manga was started around the time the LN started to hit the shelves). When the author of Dungeon Seeker was sued, he went to his fans and turned black into white and claimed Arifureta was a plagiarization of his work Dungeon Seeker and sicked his fans on the Author of Arifureta, leading to death threats and the like.
Well he won, and the courts ordered every penny of profit from the manga and ln of dungeon seeker to go to arifureta's author.