Line item disappearing on PO when entering PR

We have  a strange thing. When using ME21N and bringing a PR into a PO, the line item disappears and an error message comes up: 'Please enter line item greater than 1', however when we manually enter a line item, for example 010, the line item disappears again and gives a hard error.  We've found that if we do ME21N and manually put in line  item 010, THEN the PR number, it is OK and the PO is able to be created.

This has never happened before. What would cause it? Message 06127

Thank you.