Hi Deepanshu Malhotra,
From the description it seems that the partner functions are mapped from CRM to ECC and ECC to CRM which is the common setting. As you seem to redetermine the contact person in ECC you get the described error.
You seem to have 2 options.
Firstly to have the masterdata and customizing (classification of the BP/Contact person) in synch in CRM and ECC.
Secondly you might consider not to map the contact person from ECC to CRM, but this can be an issue as you might need it in some of your processes but not all.
Best regards - Christophe