Open user interface: Control Panel > System Settings > Foundation > FreeMarker Engine.
This interface has 2 variables, "Restricted classes" and "Restricted variables", that are list of values. There is no UI control to remove the first item in the list. Unlike item number 2 and higher, the only way to remove the first item is to make it empty using [Backspace] to delete its value. The side effect is that you end up with the value of "Restricted classes" as:
[, java.lang.ClassLoader, java.lang.Thread]
where the first value is empty instead of the more reasonable value of:
The implementation of this UI is incomplete.