Changing IAM Role on EC2 instance: Internal Error

Hello,

I am trying to change the IAM role assigned to an ec2 instance.

In the AWS console, it results in:
Error updating IAM Role
An internal error has occurred

I have tried it with multiple ec2 instances. I've tried it with new roles and roles that have worked before.

What other information can I look at to diagnose and/or fix this?

Thanks!
JerryC