We use IFS 8.  We have 2 sites, A and B.  Some time A will buy some part from site B.  Buyer released PO and Send Order. Then sales of site B will get a mail with Message ID. Sales approve this incoming CO then get a planned CO. Sales need change Wanted Delivery Date in CO Header and save it. It pops up errror message like follow picture when saving frome last week (It never happened before.).  I am sure that no other user edit this CO. Then user must quit IFS and login again. Then changing CO data and saving works. 

What cause it?

How can I prevent it happen again?

By the way, the system will create a Shop Order automatically when release CO.