In each communication point you pass the current time and the step length as argument to the fmi2DoStep function. This error message means that in the following step, the time of the communication point t_{i+1} is not exactly equal to t_{i} + delta_t. So, most likely this is a truncation error of the master algorithm.

You could replace the computation of the next communication point by a getReal call to receive the communication time computed by the FMU after each doStep call. If you cannot modify the master algorithm, you have to contact the support to let the developers fix it.