Display of 'ISV-code error' instead of custom message from PlugIn on PreStep

I developed a PlugIn which runs on the PreOperation of the update- as well as the create-step and validates the information entered by the user. If this validation fails, the user has to be notified and the update- or create-Operation needs to be aborted.

The display of my error messages is working fine on my Dev-Environment (CRM 2011 UR 13), but on one of our Test-Servers (CRM 2011 UR 11) the only message displayed to any user is: "Business Process Error: ISV code aborted the operation."

I use the following try-catch block and codeline to throw an exception:

try
{
    Bool valid = false;
    if (!valid)
    {
        throw new InvalidPluginExecutionException(OperationStatus.Failed, "Invalid information message");
    }
}
catch (InvalidPluginExecutionException e)
{
    throw e;
}
catch (Exception ex)
{
    string generalException = ex.Message + "\n\n" + ex.StackTrace;
    if (ex.InnerException != null)
    {
        generalException += "\n\nInnerException:\n" + ex.InnerException.Message + "\n\n" + ex.InnerException.StackTrace;
    }
}

I am well aware of the fact, that this issue might be solved by installing the latest UR - but as this is not an option on a few systems, I am desperately looking for a workaround.

Any help would be greatly appreciated.