Unfortunately we do not have enough information on the error you are encountering to make a suggestion here. For very specific modeling issues we request that you attach an example .mdl or .plecs file, or contact our support team at firstname.lastname@example.org.
Also, this text is from our user manual regarding the diode turn-on threshold setting:
Note The Diode Turn-On Threshold is not equivalent to the voltage drop across a device when it is conducting. The turn-on threshold only delays the instant when a device turns on. The voltage drop across a device is solely determined by the forward voltage and/or on-resistance specified in the device parameters.
I hope this helps.