While trying to import solution to our test environment from our dev environment we got the below error
“Crm Exception: Message: Customer lookup [CustomerId] is invalid. Customer lookups can only be associated with 1 Account and 1 Contact”
The way we got it fixed
Unzip the solution and open customizations.xml file
Remove all attributes with name “CustomerId” (Lead, Invoice, Opportunity entity)
Find all entity relationships with ReferencingAttributeName equals CustomerId and add to EntityRelationship new attribute: unmodified=”1″
Zip solution back and import it.
Hope it helps..