I don't know your topology but some things you can try:Originally posted in cdnusers.org by Kalevi2
a) Just drive the output into the input over a simple T line at your operating frequency. If you get the same error, remove the v-t curves and see if it works.
b) If the output into the input works, start adding the bits of your topology to see where it fails.
c) Also, try to change the time step setting to a finer time step.