I have been having the exact same problem. The error message you are getting refers to the web master's setting; there is no way for you to set the "custom error message".
I get this error almost every time after pressing the Add Next Aircraft button on the application. The solution appears to be pressing the browser's Back button, then pressing the Add Next Aircraft button. Continue to do this until you don't get the error.
This does appear to be a bit of an improvement over the errors I was getting last week while filling out the application... After selecting the Aircraft Type from the pop up menu, then selecting Add Next Aircraft, the type of aircraft you had previously selected was changed to a new, random type.
Good luck!