I had following situation. When I tried to edit and save some of custom object CustomObject1__c instance, I received error in the top of the page
Error: Invalid Data. Review all error messages below to correct your data. Operation not valid for this user type
When I investigated deeper, so I have found out that this object CustomObject1_c has lookup relation to another object CustomObject2_c for which instance of one more custom object CustomObject3_c have been trying to be inserted. Object CustomObject3_c also has lookup relation to object CustomObject2_c, so during insert object CustomObject2_c was considered to be updated with new child relationship record.
So when I tried manually to update corresponding instance of CustomObject2__c object, I saw another error instead of page
Can't Perform this Operation You can't perform this operation with the selected user type. Click here to return to the previous page.
I was trying to perform this operation under system administrator, so I supposed I had all the permissions to perform any of operations on any object.
Why did I have that error?