It's completely that Marvel wants the rights back, but they don't want to pay for it.
That's ridiculous. Why should Marvel have to pay for something they already own, and that is the IP known as the Fantastic Four? Fox has a sort of renewable lease on the rights to make films featuring the IP. Fox doesn't own it outright. Marvel management at the time of these deals didn't negotiate from a position of power since they were struggling financially. They should have had better legal counsel when putting the deal together. Some say this is why Ike Perlmutter has sort of exiled his pal Avi Arad who was in charge at that time IIRC.
Fox shot themselves in the foot by rushing something into production before the terms ended for their rights on making films using that IP. Now they've probably poisoned the well for years to come.
Notice that they only announced a film for Victor Von Doom. That's a curious notion considering that they've angered the fans with their failed versions. But there is some hope that Noah Hawley has a better idea on how to make a successful transition from comics to screen. I think the FF and its cast is a bad fit for Fox since they really don't mesh well with the X-Men. Remember all those times when Magneto faced the FF? or the X-Men faced Galactus, or Doom? You don't?