This error was triggered when accessing the user field with str[endsWith] when user is null:

java.lang.NoClassDefFoundError: mssp/io/model/user (wrong name: mssp/io/model/User)

Condition that triggers the error:
$a : Alert(user!=null, user str[endsWith] "$")

This does not trigger the error:
$a : Alert(user!=null, user matches ".+
$")

If I remove the import for the User class the error will not be triggered. As a workaround I've switched to using matches instead of str[endsWith], but I'd prefer to switch back.

I did not experience this issue in 6.3.0-Final, so it appears to be a regression in 6.4.0-Final