I have a Master-Detail relationship with Accounts and Locations.
I am able to create the child using
upsert, but when I try to update the
Account__c field I get
(INVALID_FIELD_FOR_INSERT_UPDATE: Unable to create/update fields: Account__c. Please check the security settings of this field and verify that it is read/write for your profile or permission set.)
Profile and permission set look OK, from what I can tell.