Hi,
I had the same issue previously, but 1528 did not do it. I have since changed build on that unit and can't remember the actual value I used, But it was larger.
All I did was increase the value until the warning disappeared, and everything started working as advertised.
what do you mean with "working as advertised" ? It should work even if you get this warning. All it does is fragmenting your traffic.
Setting the MTU to 1528 works for me. Without providing more information about your setup and giving us the number which seemingly "fixed" it for you nobody can help you.
Regards, Marek