unexpected error can come to n number of reasons.
Usually occurs while saving the record, but sometimes occurs even when you are trying load a record. Have faced the same multiple times, and each time the reason after through investigation with Netsuite Support comes as a new insight to us, which is not mentioned anywhere in the documentation.
Every system has its own limitation and they are built in someway, some is applied for Netsuite as well.
Your best bet to resolve these errors is to raise a case with Netsuite and they will let you know the exact reason, after which you can change how you are trying to accomplish the requirement.