The Rigid Dynamics FMU uses a server-client architecture. The FMU is a thin client layer which uses the Rigid Dynamics solver in the background. Consequently, if the FMU primary abruptly aborts the simulation, the Rigid Dynamics solver executable may be left running. This orphan process prevents further solves. You must manually kill the solver process (Ansys.Solvers.RBD.exe), and the contact server (Ans.common.ContactServer.exe) using, for instance, the Windows Task Manager.

Typically, this problem occurs with Twin Builder when the simulation is aborted with the Abort option. Always use the Clean Stop as described here.