According to the lawsuit (via Deadline), Icon is accusing Voltage Pictures of not abiding by the terms of the production contract.
In addition, one accusation of the lawsuit that seems supremely ironic, Voltage was responsible for selecting a completion guarantor, somebody who was responsible for making final decisions when the two production companies could not agree.
The lawsuit is seeking compensatory and punitive damages to be awarded, as well as for the films rights to revert exclusively to Icon Productions.
Specifically, Voltage Pictures allegedly never submitted a final budget for the production, and made changes to the shooting schedule, including not having key scenes filmed in Oxford, England, as the lawsuit claims had been previously agreed.