A change introduced in 5.2.0 changes the handling of COLDBOX_FAIL_FAST when a UDF is defined.
In coldbox\system\Bootstrap.cfc, onRequestStart() used to have a
at the end of the
block. 5.2.0 moved that return false up to the Core Fail Fast Option section.
What this means to us is that since we defined a UDF to customize the message, it will show that message and then show whatever it was attempting to which makes it invalid in our case. Our UDF is the same as the Core option just with a customized message.
I think the return false needs to go back where it was. When I made that change locally, things operated as expected again.