I talked with the OptiSystem team again and got a few updates!
You can keep working with the OFDM sample if you desire, the component should function fine but it isn’t the easiest one to use. The parameters are picky, in your case I think the change in fiber length is changing the delay of the signal, which is one of the parameters you need to edit.
Regardless of what you choose we are going to be releasing a minor update 13.0.2 (which includes bug fixes and better OFDM support) in approximately a month from now. Hopefully this will fix the issues you are experiencing! I believe you are using an evaluation license, so once the fix is released you can email me at damian.marek@optiwave.com and I can help extend your license for you.
Sorry for the inconvenience.