I have 3 custom, locked buttons that show when I am editing the page layout but when I go to create a new record using that same layout the buttons do not appear. I have checked the field security and it is correct.

I'm trying to create a new Opportunity in Sandbox mode and all of a sudden I'm getting an error message:
Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger kell01.CreateDefaultAllocation caused an unexpected exception, contact your administrator: kell01.CreateDefaultAllocation: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: a1Ei00000003MUL: []: (kell01)

What is causing this?

I'm trying to create a new Opportunity in Sandbox mode and all of a sudden I'm getting an error message:
Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger kell01.CreateDefaultAllocation caused an unexpected exception, contact your administrator: kell01.CreateDefaultAllocation: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: a1Ei00000003MUL: []: (kell01)

What is causing this?