Where to start?
The rumor that he didn't screen test MBJ? The rumor about him sexually harassing Kate Mara? The rumor about Trank costing the studio more money to repair a home he destroyed for the film than the actual film cost? Maybe you're thinking of the rumor that Trank would often not show up and when he did, he was both high and late? Then there's the rumor about him being fired in post-production after the producers saw the footage and found it to be unreleasable.
Those are the Trank rumors.
Keep in mind that a good Fantastic Four movie should be something similar to Gravity, Interstellar, Apollo 13, Alien or 2001: A Space Odyssey. Right now it looks like it's going to be more inspired by Swordfish and The Net. Fantastic Four is NOT a techno-thriller but Josh Trank wanted to make a film about the FF that was set on Earth and features the Negative Zone but with Doom as the villain instead of Annihilus. Like if you're using the Ultimate N-Zone origin, use Annihilus and have him either start an alien invasion or have him be an unintended stowaway. Now we're getting a version of Doctor Doom whose costume looks cheap amid rumors of the Doombots being cut and the confirmation that he's going to be a techno-thriller villain instead of an iron-fisted dictator/robotics genius.
This is no longer a story about astronauts with incredible powers. It's the story of how to squander a $100M budget and act like an amateur while directing a tentpole film.