In our environment, we have two custom Course objects, one's identifier is Course__c the other is hed__Course__c. (Don't ask me why, it's just how it is.)
The one I am focused on is Course__c. Course__c has a look up field that looks to another Course__c, this is for linking translated courses. In my object manager, everything looks correct, the field says the field name is Course__c.
However, when I go to assign this value through the API, I get the error Course: id value of incorrect type.
Double checking on this, through the lightning portal, I attempted to add the course field manually, it is searching for hed__Course__c, not Course__c. And when I select Add New it gives me the form to create a hed__Course__c.
Why would Salesforce get confused on this? The two objects have different names, just the same label... Seems like a bug to me
Course__cwhile the field looks up to, well, anything. You need to look at the field detail in Object Manager and click through the link to the looked-up object. You'll find it'shed__Course__cunless you've got some kind of other customization confusing things. – David Reed♦ Feb 14 '19 at 0:29