I’m not sure what the Layout parameter “Symbol” is you made, but if it is for symbol rate then I think it is incorrect. I believe I changed this in an earlier file you sent me. The filter’s bandwidth in the Optical Receiver depend on this correct value, which should be 10 Gbps, since you are using a bit rate of 10 Gbps and the modulation scheme is OOK-RZ.
I’m not sure why but the Optical Receiver is not finding the correct decision instant so I set it manually in the first receiver to 0.5 bits. It seems to be giving accurate results now, although the BER is low.